
import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();

		int a[] = new int[n];

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

		long sum = 0;

		for(int i = 1; i < n; i++){
			if(a[i] < a[i - 1]){
				sum += a[i - 1] - a[i];
				a[i] = a[i - 1];
			}
		}

		System.out.println(sum);
	}
}