import java.util.*;

public class Main {

    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        long res = 0;

        int[] a = new int[n];
        for(int i=0; i<n; i++) {
            a[i] = sc.nextInt();
        }
        int max = a[0];
        for(int i=1; i<n; i++) {
            if(max > a[i]) {
                res += max-a[i];
            }
            max = Math.max(max, a[i]);
        }
        System.out.println(res);


    }


}