#include<bits/stdc++.h> using namespace std; int main() { long long n; cin>>n; vector<long long>a(n+1); for (int i = 1; i <= n; i++) { cin >> a[i]; } if (n == 1) { cout << 0 << endl; return 0; } vector<long long>d(n); for (int i = 1; i < n; i++) { d[i] = a[i+1] - a[i]; } vector<long long> p(n + 1, 0); for (int i = 1; i < n; i++) { p[i+1] = p[i] + max(0LL, 1 - d[i]); } vector<long long>s(n + 1, 0); for (int i = n - 1; i >= 1; i--) { s[i] = s[i+1] + max(0LL, d[i] + 1); } long long minn = -1; for (int k = 1; k <= n; k++) { long long curr = max(p[k], s[k]); if (minn == -1 || curr < minn) { minn = curr; } } cout << minn << endl; }