#include<stdio.h>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>
#include <sstream>
#include <iostream>
#include <string>
#include <stdio.h>

using namespace std;

int main() {

	int N;
	long long C, D, sum = 0;

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

	cout << sum << endl;

	return 0;
}