Duff is addicted to meat! Malek wants to keep her happy for *n* days. In order to be happy in *i*-th day, she needs to eat exactly *a*<sub class="lower-index">*i*</sub> kilograms of meat.

There is a big shop uptown and Malek wants to buy meat for her from there. In *i*-th day, they sell meat for *p*<sub class="lower-index">*i*</sub> dollars per kilogram. Malek knows all numbers *a*<sub class="lower-index">1</sub>, ..., *a*<sub class="lower-index">*n*</sub> and *p*<sub class="lower-index">1</sub>, ..., *p*<sub class="lower-index">*n*</sub>. In each day, he can buy arbitrary amount of meat, also he can keep some meat he has for the future.

Malek is a little tired from cooking meat, so he asked for your help. Help him to minimize the total money he spends to keep Duff happy for *n* days.
The first line of input contains integer *n* (1 ≤ *n* ≤ 10<sup class="upper-index">5</sup>), the number of days.

In the next *n* lines, *i*-th line contains two integers *a*<sub class="lower-index">*i*</sub> and *p*<sub class="lower-index">*i*</sub> (1 ≤ *a*<sub class="lower-index">*i*</sub>, *p*<sub class="lower-index">*i*</sub> ≤ 100), the amount of meat Duff needs and the cost of meat in that day.
days = int(input())

kgNeed = []
kgPrice = []
kgAcc = 0
amount = 0
totalNeeded = 0
for i in range(days):
  need, price = [*map(int, input().split())]
  kgNeed.append(need)
  kgPrice.append(price)
  totalNeeded += need

for i in range(days - 1):
  precoHoje = kgPrice[i]
  precoAmanha = kgPrice[i + 1]
  kgNeedHoje = kgNeed[i]
  kgNeedAmanha = kgNeed[i + 1]

  if(precoHoje == 1):
    soma = sum(kgNeed[i:len(kgNeed)])
    kgFaltante = soma - kgAcc
    amount += kgFaltante
    break  

  if(precoHoje > precoAmanha and kgAcc - kgNeedHoje < 0):
    #print("cond1")
    comprarKg = kgNeedHoje - kgAcc
    kgAcc = 0
    amount += comprarKg * precoHoje
    #print(amount)
  #elif(precoHoje > precoAmanha and kgAcc - kgNeedHoje > 0):
    #print("cond2")
    #nao compra hoje
    #print()
    #print(amount)
  elif(precoHoje < precoAmanha):
    #print("cond3")
    comprarKg = (kgNeedHoje + kgNeedAmanha - kgAcc)
    if(comprarKg > 0):
      kgAcc = comprarKg
      amount += kgPrice[i] * comprarKg
      #print(amount)

print(amount)   

'''
input
3
1 3
2 2
3 1
output
10
input
3
1 3
2 1
3 2
output
8
'''