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 ans = 0;
    for(int i = 1; i < n; i++){
      if(a[i-1] > a[i]){
        ans += a[i-1]-a[i];
        a[i] = a[i-1];
      }
    }
    
    
      System.out.println(ans);
    
  }
}