import java.util.ArrayList;
import java.util.Scanner;

public class Main {
	public static void main(String args[]) {
		Scanner stdIn = new Scanner(System.in);
		int N = stdIn.nextInt();
		ArrayList<Integer> A = new ArrayList<Integer>();
		for(int i = 0; i < N; i ++) {
			A.add(stdIn.nextInt());
		}
		
		long stepSum = 0;
		
		for(int i = 1; i < N; i ++) {
			if(A.get(i - 1) <= A.get(i)) {
				continue;
			}else {
				stepSum += A.get(i - 1) - A.get(i);
				A.set(i, A.get(i - 1));
			}
		}
		
		System.out.println(stepSum);
	}
}
/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