import java.math.BigInteger;
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 stool = 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]){
        		stool += a[i-1] - a[i];
        		a[i] = a[i-1];
        	}
        }
        System.out.println(stool);

  	}
}

