#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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