#!/usr/bin/env python3
#fileencoding: utf-8

N = int(input())

count = 0
for i in range(N):
    C, D = [int(i) for i in input().strip().split(" ")]
    if C % 2 == 0:
        count += C//2 * D
    else:
        count += (C//2+1) * D

print(count)