#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

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