#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
#define rep(i,a) for(int i=0;i<(a);i++)
#define MOD 1000000007

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