import java.util.*;

public class Main{
        public static void main(String[] args){
                Scanner sc = new Scanner(System.in);
                String str = sc.nextLine();
        int N = Integer.parseInt(str);

                long count=0;
        int prev_person = sc.nextInt();

        for(int i = 1; i < N; i++){
            int now_person = sc.nextInt();
            if(now_person < prev_person){
                count += prev_person - now_person;
            }
            else{
                prev_person = now_person;
            }
        }


                System.out.println(count);
                sc.close();
        }
}


/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