import java.util.Scanner;

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

		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		Long[] a = new Long[n + 1];
		for(int i = 0 ; i < n ; i++) {
			a[i] = sc.nextLong();
		}
		long max = 0;
		long ans = 0;
		for(int i = 0 ; i < n ; i++) {
			if(max > a[i]) {
				ans += max - a[i];
			}
			if(max < a[i]) {
				max = a[i];
			}
		}
		sc.close();
		System.out.println(ans);

	}
}


/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz