You are given a graph with $n$ nodes and $m$ directed edges. One lowercase letter is assigned to each node. We define a path's value as the number of the most frequently occurring letter. For example, if letters on a path are "abaca", then the value of that path is $3$. Your task is find a path whose value is the largest.
The first line contains two positive integers $n, m$ ($1 \leq n, m \leq 300\,000$), denoting that the graph has $n$ nodes and $m$ directed edges.

The second line contains a string $s$ with only lowercase English letters. The $i$-th character is the letter assigned to the $i$-th node.

Then $m$ lines follow. Each line contains two integers $x, y$ ($1 \leq x, y \leq n$), describing a directed edge from $x$ to $y$. Note that $x$ can be equal to $y$ and there can be multiple edges between $x$ and $y$. Also the graph can be not connected.
In the first sample, the path with largest value is $1 \to 3 \to 4 \to 5$. The value is $3$ because the letter 'a' appears $3$ times.
5 4
abaca
1 2
1 3
3 4
4 5
3
6 6
xzyabc
1 2
3 1
2 3
5 4
4 3
6 4
-1
10 14
xzyzyzyzqx
1 2
2 4
3 5
4 5
2 6
6 8
6 5
2 10
3 9
10 9
4 6
1 10
2 8
3 7
4
1 1
f
1 1
-1
10 50
ebibwbjihv
1 10
1 2
5 4
1 8
9 7
5 6
1 8
8 7
2 6
5 4
1 9
3 2
8 3
5 6
5 9
2 4
2 7
3 9
1 2
1 7
1 10
3 7
1 8
3 10
8 6
1 7
10 6
1 6
5 8
1 5
2 10
3 9
5 8
8 3
3 7
5 2
1 10
1 4
5 3
3 2
1 2
5 8
10 4
2 10
8 2
1 9
1 8
1 2
3 4
1 8
2
13 37
dwpzcppjmhkmz
2 6
3 6
6 7
6 7
6 7
6 7
6 8
6 8
6 8
6 8
4 6
4 6
5 6
4 6
4 6
6 9
6 9
6 10
6 10
6 10
6 10
4 6
1 6
1 6
10 11
6 11
1 6
6 12
6 12
6 12
6 13
6 13
6 13
6 13
3 6
2 6
2 6
3
5 8
tetqw
2 1
4 4
5 5
5 2
4 5
1 5
1 5
1 1
-1
5 8
reeet
4 3
2 5
4 2
2 4
4 2
5 2
3 3
3 4
-1
from builtins import str
from typing import Dict, List, Optional, Set, Tuple


class GraphPath:
    def __init__(
        self, nodes: Set[str] = set(), letter_counts: Dict[str, int] = dict()
    ) -> None:
        self.nodes = nodes
        self.letter_counts = letter_counts

    def __repr__(self) -> str:
        return "nodes={}, letters={}".format(self.nodes, self.letter_counts)


def get_max_value_string_helper(
    graph_path: GraphPath, node: str, adjacency_map: Dict[str, Set[str]]
) -> List[GraphPath]:
    if node in graph_path.nodes:
        return [graph_path]

    new_nodes = graph_path.nodes.copy()
    new_nodes.add(node)
    new_letter_counts = graph_path.letter_counts.copy()
    if node[0] not in new_letter_counts:
        new_letter_counts[node[0]] = 0
    new_letter_counts[node[0]] += 1

    new_graph_path = GraphPath(new_nodes, new_letter_counts)

    if node not in adjacency_map:
        return [new_graph_path]

    paths = list()
    for child_node in adjacency_map[node]:
        new_paths = get_max_value_string_helper(
            new_graph_path, child_node, adjacency_map
        )
        paths.extend(new_paths)
    return paths


def get_max_value_string(
    graph_string: str, edge_list: List[Tuple[int, int]]
) -> Optional[int]:
    letter_counts = dict()
    nodes = list()
    for char in graph_string:
        if char not in letter_counts:
            letter_counts[char] = 0
        else:
            letter_counts[char] += 1
        nodes.append("{}{}".format(char, letter_counts[char]))

    adjacency_map = dict()
    for start, end in edge_list:
        start = start - 1
        end = end - 1
        if nodes[start] not in adjacency_map:
            adjacency_map[nodes[start]] = set()
        if nodes[start] != nodes[end]:
            adjacency_map[nodes[start]].add(nodes[end])

    paths = list()
    graph_path = GraphPath()
    for node in adjacency_map:
        new_paths = get_max_value_string_helper(graph_path, node, adjacency_map)
        paths.extend(new_paths)

    max_value = (0, {})
    for path in paths:
        max_path_value = max(path.letter_counts.values())
        if max_path_value > max_value[0]:
            max_value = (max_path_value, path.letter_counts)

    if len(set(max_value[1].values())) == 1:
        return -1

    return max_value[0] if max_value[0] > 0 else -1


if __name__ == '__main__':
    n_and_m = input()
    n = int(n_and_m.split(' ')[0])
    m = int(n_and_m.split(' ')[1])
    str = input()
    edges = []
    for i in range(m):
        current_line_elements = input().split(' ')
        edges.append((int(current_line_elements[0]), int(current_line_elements[1])))

    print(get_max_value_string(str, edges))
