import java.io.*;
import java.util.*;


public class Main_yukicoder633 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		int n = sc.nextInt();

		int[] a = new int[n - 1];
		for (int i = 0; i < n - 1; i++) {
			a[i] = sc.nextInt();
		}

		int[] b = new int[n];
		int[] c = new int[n];
		for (int i = 0; i < n; i++) {
			b[i] = sc.nextInt();
			c[i] = sc.nextInt();
		}

		int ans = 0;
		int num = 0;
		for (int i = 0; i < n; i++) {
			if (i > 0) {
				ans += num * a[i - 1];
			}
			num += c[i];
			num -= b[i];
		}

		pr.println(ans);
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(System.in);
		pr = new Printer(System.out);

		solve();

		pr.close();
		sc.close();
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}