#include <iostream>
#include <algorithm>
using namespace std;

int main(){
    int n;
    cin >> n;
    int cost[n - 1];
    for(int i = 0; i < n - 1; i++){
        cin >> cost[i];
    }
    int member;
    cin >> member >> member;
    int ans = 0;
    for(int i = 0; i < n - 1; i++){
        ans += member * cost[i];
        int minus, plus;
        cin >> minus >> plus;
        member += plus - minus;
    }
    cout << ans << endl;
}