#include <iostream>
#include <sstream>
#include <math.h>
#include <stack>
#include <set>
#include <algorithm>
#include <string>
#include <queue>
#include <stack>
#include <map>
#include <cstdio>
#include <vector>
using namespace std;

#define INF (1 << 30)
#define INFLL (1LL << 60)

int main() {
	long long int n,c,d;
	cin >> n;
	long long int ans = 0;
	for(int i = 0;i < n;i++){
		cin >> c >> d;
		if(c % 2 == 0){
			ans += c/2 * d;
		}else{
			ans += (c/2+1) * d;
		}
	}
	cout << ans << endl;
	return 0;
}