#include <bits/stdc++.h>
using namespace std;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n; cin >> n;
long long a[n], suma[n][2], odp = 1000000000000000000;
for (int i = 0; i < n; i++)
cin >> a[i];
suma[0][0] = suma[n-1][1] = 0;
for (int i = 1; i < n; i++)
suma[i][0] = suma[i-1][0] + ((a[i-1] >= a[i]) ? a[i-1]-a[i]+1 : 0);
for (int i = n-2; i >= 0; i--)
suma[i][1] = suma[i+1][1] + ((a[i] <= a[i+1]) ? a[i+1]-a[i]+1 : 0);
for (int i = 0; i < n; i++)
odp = min(odp,max(suma[i][0],suma[i][1]));
cout << odp << endl;
return 0;
}
