/* -*- coding: utf-8 -*-
 *
 * 633.cc: No.633 バスの運賃 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 100;

/* typedef */

typedef vector<int> vi;
typedef queue<int> qi;
typedef pair<int,int> pii;

/* global variables */

int as[MAX_N], bs[MAX_N], cs[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);

  for (int i = 0; i < n - 1; i++) scanf("%d", &as[i]);
  for (int i = 0; i < n; i++) scanf("%d%d", &bs[i], &cs[i]);

  int k = 0, sum = 0;
  for (int i = 0; i < n - 1; i++) {
    k += cs[i] - bs[i];
    sum += as[i] * k;
  }
  printf("%d\n", sum);
  return 0;
}