#include <bits/stdc++.h>

using namespace std;

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,b,e) for (int i=(int)(b); i<(int)(e); i++)
#define ALL(x) (x).begin(), (x).end()

const double PI = acos(-1);

int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);

  int n;
  cin >> n;
  vector<int> a(n-1);
  REP (i, n-1) cin >> a[i];

  int ret = 0;
  int acc = 0;
  REP(i, n) {
    int b, c;
    cin >> b >> c;
    ret += acc * (b - c);
    acc += a[i];
  }
  cout << ret << endl;
  
  return 0;
}