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

int main() {
  int n;
  cin>>n;
  int a[n-1];
  for (int i=0; i<n-1; i++) cin>>a[i];
  int n_passengers = 0;
  int ans = 0;
  for (int i=0; i<n; i++) {
    int b, c;
    cin>>b>>c;
    if (i > 0) {
      ans += a[i-1] * n_passengers;
    }
    n_passengers = n_passengers - b + c;
  }
  cout<<ans<<endl;
}