import java.util.*;
public class Main{
    public static void main(String[]args){
        Scanner scn=new Scanner(System.in);
        int n=scn.nextInt();
        if(n<=3){
            System.ouyt.print("Invalid input");
            return;
        }
        int height[]=new int[n];
        for(int i=0;i<n;i++){
            height[i]=scn.nextInt();
        }
        int left=0;
        int right=height.length-1;
        int water=0;
        int leftmax=0;
        int rightmax=0;
        while(left<right){
            leftmax=Math.max(leftmax,height[left]);
            rightmax=Math.max(rightmax,height[right]);
            if(leftmax<rightmax){
                water+=leftmax-height[left];
                left++;
            }
            else{
                water+=rightmax-height[right];
                right--;
            }
        
        }
                    System.out.print(water);

    }
}