#include<iostream>
#include<vector>
#include<algorithm>
#include<string>
#include<cmath>
#include<cstdio>
using namespace std;
int main(){
    int N,sum=0,c,d;
    cin >> N;
    while(N--){
        cin >> c >> d;
        sum += d*((c+1)/2);
    }
    cout << sum;
    return 0;
}