#include <bits/stdc++.h>
using namespace std;
int main()
{
int n;
cin >> n;
vector<int> a(n);
for(int i = 0; i < n; i++){
cin >> a[i];
}
int mx = a[0] - 0;
int mn = a[0] - 0;
for(int i = 0; i < n; i++){
mx = max(mx, a[i] - i);
mn = min(mn, a[i] - i);
}
cout << mx - mn << endl;
return 0;
}
