import java.util.Arrays;
import java.util.Scanner;

class Main {
	public static void main(String[] args) throws Exception {
		new Main().run();
	}
	
	void run() {
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		int[] a=new int[n-1];
		int[] b=new int[n];
		int[] c=new int[n];
		int ans=0;
		int num=0;
		for(int i=0;i<n-1;++i)a[i]=sc.nextInt();
		for(int i=0;i<n;++i) {
			b[i]=sc.nextInt();
			c[i]=sc.nextInt();
			num+=c[i]-b[i];
			if(num>0)
				ans+=num*a[i];
		}
		System.out.println(ans);
	}
	
	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
	
}