import java.util.*;

public class Main {
    public static void main(final String[] args) {
        final Scanner sc = new Scanner(System.in);
        // 整数の入力
        // final int a = sc.nextInt();
        // 文字列の入力
        // final String s1 = sc.next();
        final int varN = Integer.parseInt(sc.next());
        int lastA = Integer.parseInt(sc.next());
        int presentA = 0;
        long cnt = 0;
        for (int i = 1; i < varN; i++) {
            presentA = Integer.parseInt(sc.next());
            if (presentA < lastA) {
                cnt = cnt + lastA - presentA;
                continue;
            } else {
                lastA = presentA;
            }
        }
        sc.close();

        System.out.println(cnt);
    }
}
/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