#include <iostream>
#include <algorithm>
#include <cstring>
#include <climits>
#define N_MAX 100000
#define M_NUM 1000000007
using namespace std;
typedef long long LL;

int main()
{
	int N;
	LL C, D, ans=0;
	cin >> N;

	for (int i = 0; i < N; ++i)
	{
		cin >> C >> D;
		ans += ((C+1) / 2)*D;
	}

	cout << ans;

	return 0;
}