import java.util.*;

 
public class Main {
	public static void main(String[] args) {
		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();
		}

		int count = 0;

		for(int i = 0; i < n - 1; i ++){
			if(a[i] > a[i + 1]){
				count += (a[i] - a[i + 1]); 
				a[i + 1] = a[i];
				
			}
		}
		System.out.println(count);

	}
}