import java.util.*;
public class Main {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        long[] a = new long[n];
        for(int i = 0; i < n; i++){
            a[i] = sc.nextInt();
        }
        long len = 0;
        for(int i = 1; i < n; i++){
            if(a[i-1] > a[i]){
                len += a[i-1] - a[i];
                a[i] += a[i-1]-a[i];
            }
        }
        System.out.println(len);
    }
}