import java.util.Scanner;

public class Main {

	static int ans;

	public static void main(String[] args) {

		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = new int[n];
		long sum = 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]) {
				sum = sum + a[i-1] - a[i];
				a[i] = a[i-1];
			}
			
		}
		
		System.out.println(sum);
	}
}