#define _GLIBCXX_DEBUG
#define ll long long
#define INF 1000000000;
#define INF_B 1000000000000000000;
#include <bits/stdc++.h>
using namespace std;
using Graph = vector<vector<int>>;

int main(){
  int N; cin >> N;
  ll ans = 0;
  for(int i = 0; i < N; i++){
    ll C, D; cin >> C >> D;
    C = (C+1)/2;
    ans += C*D;
  }
  cout << ans << endl;
}