import java.util.*;

public class Main{
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int loop = sc.nextInt();
        int[] array = new int[loop];
        int count = 0;
        for(int i = 0; i < loop; i++) {
            array[i] = sc.nextInt();
        }
        if(loop == 1){
            System.out.println("0");
        }
        else{
            for(int i = 0; i < (loop - 1); i++) {
                int temp = array[i];
                int next = array[i+1];
                if(next < temp){
                    count += (temp - next);
                    array[i+1] = temp;
                }
            }
            System.out.println(count);
        }

    }
}
