#include <bits/stdc++.h> #define ll long long using namespace std; const ll N = 1e6, mod = 1e9 + 7, inf = 1e18; int main() { ios_base::sync_with_stdio(false); cin.tie(nullptr); int n; cin >> n; vector<ll> a(n + 2, 0); for (int i = 1; i <= n; i++) { cin >> a[i]; } vector<ll> k(n + 2, 0), d(n + 2, 0), d1(n + 2, 0); ll ans = 0; vector<ll> b = a; for (int i = n; i >= 1; i--) { if (b[i] + d[i + 1] > b[i + 1]) { k[i] = ans; d[i] = d[i + 1]; b[i] = b[i] + d[i + 1]; } else { b[i] = b[i] + d[i + 1]; ll e = b[i + 1] - b[i] + 1 + d[i + 1]; d[i] = max(e, d[i + 1]); ans += d[i] - d[i + 1]; b[i] = b[i + 1] + 1; k[i] = ans; } } b = a; ll mnd = int64_t(1e18); ans = 0; for (int i = 1; i <= n; i++) { if (b[i] + d1[i - 1] > b[i - 1]) { d1[i] = d1[i - 1]; b[i] = b[i] + d1[i - 1]; } else { b[i] = b[i] + d1[i - 1]; ll e = b[i - 1] - b[i] + 1 + d1[i - 1]; d1[i] = max(e, d1[i - 1]); ans += d1[i] - d1[i - 1]; b[i] = b[i - 1] + 1; } mnd = min(mnd, max(ans, k[i])); } cout << mnd << '\n'; }