#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;

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