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];
		int step=0;
		for(int i=0;i<N;i++){
			A[i]=sc.nextInt();
		}
		for(int i=1;i<=N;i++){
			if(A[i-1]>A[i]){
				step+=A[i-1]-A[i];
				A[i]=A[i-1];
			}
		}
		System.out.println(step);
	}	
}
			