# -*- coding: utf-8 -*-
#No.146 試験監督（1）
__author__ = 'yuuki'

N = input()
a =[]
for i in xrange(N):
    a.append(map(int,raw_input().split()))
zyukou = 0

for i in xrange(N):
    if a[i][0] % 2 == 0:
        zyukou += (a[i][0] / 2) * a[i][1]
    else:
        zyukou += (a[i][0] / 2 + 1) * a[i][1]


print zyukou