import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        long count = 0;
        int base = 0;
        int now = sc.nextInt();
        for (int i = 0; i < N-1; i++) {
            int Ai = sc.nextInt();
            base = Ai;
            if (Ai < now) {
                base = now;
                count += (now-Ai);
            }
            now = base;
        }
        System.out.println(count);
    }
}
/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