// #include <bits/stdc++.h>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <map>
#include <vector>
#include <iomanip>
#include <string>
#include <queue>
#include <set>
#include <deque>
#include <numeric>
#include <stack>
#include <chrono>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
void fast_io() {
// freopen("", "r", stdin);
// freopen("", "w", stdout);
ios::sync_with_stdio(0);
cin.tie();
cout.tie();
cout << setprecision(9);
}
#define int long long
#define endl '\n'
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define fi first
#define se second
#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>
void solve() {
int n; cin >> n;
vector<int> a(n), l(n, 0), r(n, 0);
for (auto &i : a) cin >> i;
int cur = a[0];
for (int i = 1; i < n; i++){
cur = max(cur + 1, a[i] + l[i - 1]);
l[i] = max(l[i - 1], cur - a[i]);
}
reverse(all(a));
cur = a[0];
for (int i = 1; i < n; i++){
cur = max(cur + 1, a[i] + r[i - 1]);
r[i] = max(r[i - 1], cur - a[i]);
}
reverse(all(r));
int ans = 1e18;
for (int i = 0; i < n; i++){
ans = min(ans, max(l[i], r[i]));
}
cout << ans << endl;
}
signed main() {
fast_io();
srand(chrono::steady_clock::now().time_since_epoch().count());
int tc = 1;
// cin >> tc;
while (tc--) solve();
return 0;
}
