#include <bits/stdc++.h>
using namespace std;

signed main() {
  ios::sync_with_stdio(false);
  int N;
  cin >> N;
  vector<int> A(N - 1);
  for (int i = 0; i + 1 < N; ++i) {
    cin >> A[i];
  }
  int s = 0;
  int b0, c0;
  cin >> b0 >> c0;
  int t = c0;
  for (int i = 1; i < N; ++i) {
    int b, c;
    cin >> b >> c;
    s += t * A[i - 1];
    t = t - b + c;
  }
  cout << s << endl;
  return 0;
}