#include<iostream>
#include<cmath>

using namespace std;

int main(){
    int n;
    long long ans = 0;
    cin >> n;
    double c[n],d[n];
    for(int i=0;i < n;i++){
        cin >> c[i] >> d[i];
        ans += ceil(c[i]/2) * d[i];
    }
    cout << ans << endl;
    return 0;
}