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

int main() {
    int n;
    cin >> n;
    int a[100] = {0};
    for(int i = 1; i < n; i++) {
        cin >> a[i];
    }
    int b, c, sum = 0, ans = 0;
    for(int i = 1; i <= n; i++) {
        cin >> b >> c;
        sum += c - b;
        ans += sum * a[i];
    }
    cout << ans << '\n';
    return 0;
}