import java.math.BigInteger;
import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    BigInteger sum = BigInteger.ZERO;

    long a1 = sc.nextInt();
    for (int i = 1; i < n; i++) {
      long a2 = sc.nextLong();
      if (a1 > a2) {
        long x = a1 - a2;
        sum = sum.add(BigInteger.valueOf(x));
        a1 = a2 + x;
      } else {
        a1 = a2;
      }
    }
    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