import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO 自動生成されたメソッド・スタブ
		Scanner sc = new Scanner (System.in);
		
		long n =sc.nextInt();
		
		int height1=sc.nextInt();
		int height2=0;
		
		int takadai=0;
		long sum=0;
		
		for(int i=0;i<(n-1);i++) {
			height2 = sc.nextInt();
			if(height1>height2) {
				takadai = height1 - height2;
				sum = sum + takadai;
			}else {
				height1 = height2; 
			}
		}
		
		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