package yukicoder;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.StringTokenizer;

public class P633 {
	
	public static void main(String[] args) throws IOException {
		MyScanner sc = new MyScanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n-1];
		for(int i=0; i<n-1; i++) {
			a[i] = sc.nextInt();
		}
		int[] d = new int[n-1];
		int b = sc.nextInt();
		int c = sc.nextInt();
		d[0] = c;
		for(int i=1; i<n-1; i++) {
			int b1 = sc.nextInt();
			int c1 = sc.nextInt();
			d[i] = d[i-1]+c1-b1;
		}
		int ans = 0;
		for(int i=0; i<n-1; i++) {
			ans += d[i]*a[i];
		}
		System.out.println(ans);
	}

	static class MyScanner
	{
		BufferedReader br;
		StringTokenizer st;
		public MyScanner(InputStream s)
		{
			br=new BufferedReader(new InputStreamReader(s));
		}
		public String nextLine() throws IOException
		{
			return br.readLine();
		}
		public String next() throws IOException
		{
			while(st==null || !st.hasMoreTokens())
				st=new StringTokenizer(br.readLine());
			return st.nextToken();
		}
		public int nextInt() throws IOException
		{
			return Integer.parseInt(next());
			
		}
		public double nextDouble() throws IOException
		{
			return Double.parseDouble(next());
		}
		public boolean ready() throws IOException
		{
			return br.ready();
		}
		public long nextLong() throws IOException
		{
			return Long.parseLong(next());
		}
	}
}