# -*- coding: utf-8 -*-
"""
No.146 試験監督（1）
https://yukicoder.me/problems/no/146

"""
import sys
from sys import stdin
input = stdin.readline


def main(args):
    ans = 0
    N = int(input())
    for _ in range(N):
        C, D = map(int, input().split())
        capacity = (C + 1) // 2
        ans += capacity * D
    print(ans)


if __name__ == '__main__':
    main(sys.argv[1:])