import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Scanner;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        Task solver = new Task();
        solver.solve(1, in, out);
        out.close();
    }

    static class Task {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            final int n = in.nextInt();
            int[] a = new int[n - 1];
            for (int i = 0; i < n - 1; i++) {
                a[i] = in.nextInt();
            }
            int[] bc = new int[n];
            for (int i = 0; i < n; i++) {
                int b = in.nextInt();
                int c = in.nextInt();
                bc[i] = c - b;
            }

            int ans = 0;
            int p = bc[0];
            for (int i = 1; i < n; i++) {
                ans += a[i - 1] * p;
                p += bc[i];
            }
            out.println(ans);
        }

    }
}