#include <bits/stdc++.h> using namespace std; using ll = long long; const int N = 2e5 + 5, inf = 2e9; int n, d[N]; ll ans = 1e18, pf[N], sf[N]; int main() { ios_base::sync_with_stdio(0); cin.tie(0); // freopen(".INP", "r", stdin); // freopen(".OUT", "w", stdout); cin >> n; for (int i=1;i<=n;++i) cin >> d[i]; for (int i=n;i>=1;--i) d[i] -= d[i - 1]; for (int i=2;i<=n;++i) pf[i] = pf[i - 1] + max(0, 1 - d[i]); for (int i=n;i>=2;--i) sf[i] = sf[i + 1] + max(0, d[i] + 1); for (int i=1;i<=n;++i) ans = min(ans, max(pf[i], sf[i + 1])); cout << ans; }