#include <iostream>
#include <algorithm>
#include <vector>
#include <map>
#include <string>
#include <numeric>
#include <cstring>
#include <cmath>
#include <set>
#include <queue>
#include <stack>

const int MOD = 1e9 + 7;
const int iINF = 1000000000;
const long long int llINF = 1000000000000000000;
#define rep(i, n) for (int i = 0; i < (n); i++)

using namespace std;
using ll = long long int;
using P = pair<int, int>;
using edge = struct
{
    int to;
    int cost;
};

int main()
{
    int N;
    cin >> N;
    vector<int> a(N);
    vector<int> b(N);
    vector<int> c(N);
    for (int i = 1; i < N; i++)
        cin >> a[i];
    rep(i, N) cin >> b[i] >> c[i];

    a[0] = 0;

    int psum = 0;
    int ans = 0;
    rep(i, N)
    {
        ans += psum * a[i];
        psum += c[i];
        psum -= b[i];
    }

    cout << ans << endl;
    return 0;
}