#include <bits/stdc++.h>
#define PI 3.14159265359
#define defmod 1000000000+7
using namespace std;

int main() {
	int N;
	cin >> N;

	int cnt = 0;
	for (int i = 0; i < N; i++) {
		int a, b;
		cin >> a >> b;

		cnt += (a + 1) / 2 * b;
	}

	cout << cnt << endl;
}