#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define REP(i, n) rep(i, 0, n)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define int long long
#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
using namespace std;
typedef pair<int, int> P;
const int mod = 1000000007;
const int INF = 1e12;

signed main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    vector<int> a(n), b(n), c(n);
    rep(i, 0, n - 1){
        cin >> a[i];
    }
    rep(i, 0, n){
        cin >> b[i] >> c[i];
    }
    int now = c[0], ans = 0;
    rep(i, 1, n){
        ans += now * a[i - 1];
        now = now - b[i] + c[i];
        // cout << ans << " " << now << endl;
    }
    cout << ans << endl;
}