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];

		long ans = 0L;

		for(int i = 0; i<N; i++) {
			A[i] = sc.nextLong();
		}

		for(int i = 0; i<N-1; i++) {
			long x =A[i+1] - A[i];
			if(x<0) {
				ans -= x;
				A[i+1] -=x;
			}
		}

		System.out.println(ans);
		sc.close();
	}
}

/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