#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;


int main(){
  int N;
  cin >> N;
  vector<int> v(N, 0);
  rep(i,N-1){
    cin >> v[i+1];
  }

  int sum = 0;
  int now = 0;
  rep(i,N){
    int b, c;
    cin >> b >> c;
    sum += v[i] * now;
    now -= b;
    now += c;
  }

  cout << sum << endl;
  
  return 0;
}