import java.util.*;

public class Main{
  public static void main(String[] args){
  	Scanner sc = new Scanner(System.in);
    
    int n = sc.nextInt();
    int[] a = new int[n];
    for(int i = 0; i < n; i++){
      a[i] = sc.nextInt();
    }
    
    int sum = 0;
    for(int i = 1; i < n; i++){
	  if(a[i-1] > a[i]){
        sum += a[i-1] - a[i];
        a[i] = a[i-1];
      }
    }
    
    System.out.println(sum);
  }
}
