#include <bits/stdc++.h>
#define rep(i, ss, ee) for (int i = ss; i < ee; ++i)
using namespace std;

void solve() {
  int N, c, d, ans = 0;
  cin >> N;

  rep(i, 0, N) {
    cin >> c >> d;
    ans += (c + 1) / 2 * d;
  }
  cout << ans << endl;
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  solve();
  getchar();
}