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

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    
    int N;
    cin >> N;
    int t;
    vector<int> a;
    int ans = 0;
    
    for (int i = 0; i < N-1; ++i) {
        cin >> t;
        a.push_back(t);
    }
    
    int v;
    cin >> t >> v;
    for (int i = 1; i < N; ++i) {
        ans += a[i-1] * v;
        cin >> t;
        v -= t;
        cin >> t;
        v += t;
    }
    
    cout << ans << endl;
    
    return 0;
}