import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.math.BigInteger;

public class Main{
	public static void main(String[] args)throws Exception{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int N = Integer.parseInt(br.readLine());
		String[] strs  = br.readLine().split(" ");
		
		int max = 0;
		BigInteger sum = new BigInteger("0");
		for(String str : strs){
			int n = Integer.parseInt(str);
			if(n < max){
				sum =  sum.add(new BigInteger("" + (max - n)));
			}else{
				max = n;
			}
		}
		
		System.out.println(sum);
	}
}

/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz