#include<bits/stdc++.h>
using namespace std;
 
int main() {
    int n,lim,maxwater=0;
    cin>>n;
    int arr[n];
    for(int i=0;i<n;i++) {
    	cin>>arr[i];
    }
    int left[n],right[n];
    left[0]=arr[0];
    right[n-1]=arr[n-1];
    for(int i=1;i<n;i++) {
    	left[i]=max(left[i-1],arr[i]);
    	right[n-i-1]=max(right[n-i],arr[n-i-1]);
    }
    for(int i=1;i<n-1;i++) {
        lim=min(left[i],right[i]);
        if(arr[i]<lim) {
        	maxwater+=lim-arr[i];
        }
    }
    cout<<maxwater;
	return 0;
}

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