#include "bits/stdc++.h"
using namespace std;

int main(){
    
    int n;
    cin >> n;
    int fare[100] = {0};
    
    for(int i = 1; i<n ; i++){
        cin >> fare[i];
    }
    int out, come, passenger = 0, pay = 0;
    for(int i = 1; i<=n ; i++){
        cin >> out >> come;
        passenger += come - out;
        pay += passenger*fare[i];
    }
    cout << pay << "\n";
    return 0;
}