import java.util.Scanner;
public class Main
{
    public static void main(String[] args)
    {
        int sl=0;
        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();
        }
        for(int j=0;j<n-1;j++)
        {
            if(a[j]>=a[j+1])
            {
                sl=sl+(a[j]-a[j+1]);
                a[j+1]=a[j+1]+(a[j]-a[j+1]);
            }
            
        }
        System.out.println(sl);
    }
}