#include<iostream>
#include<vector>
#include<bitset>
#include<algorithm>
#include<cmath>
#include<string>
using namespace std;

int main(){
    int n,people=0,ans=0;
    cin >> n;
    vector <int> cost(n-1);
    for(int i=0;i<n-1;i++)  cin >> cost[i];
    vector <int> b(n),c(n);
    for(int i=0;i<n;i++)    cin >> b[i] >> c[i];
    for(int i=0;i<n-1;i++){
        people+=(c[i]-b[i]);
        ans+=(people*cost[i]);
    }
    cout << ans << endl;
    return 0;
}