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