#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<pii> vpi;
typedef vector<pll> vpl;
#define sz(x) (int)x.size()
#define all(x) x.begin(), x.end()
#define pb push_back
const int MAXN = (int)2e5 + 5;
int N;
ll a[MAXN], l[MAXN], r[MAXN], maxL[MAXN], maxR[MAXN];
int main()
{
cin >> N;
for(int i = 1; i <= N; i++)
cin >> a[i];
for(int i = 1; i <= N; i++)
{
ll free = l[i - 1] - a[i - 1];
l[i] = max(a[i] + free, l[i - 1] + 1);
}
for(int i = 1; i <= N; i++)
{
l[i] -= a[i];
maxL[i] = max(maxL[i - 1], l[i]);
}
for(int i = N; i >= 1; i--)
{
ll free = r[i + 1] - a[i + 1];
r[i] = max(a[i] + free, r[i + 1] + 1);
}
for(int i = N; i >= 1; i--)
{
r[i] -= a[i];
maxR[i] = max(maxR[i + 1], r[i]);
}
ll ans = maxL[N];
for(int i = 1; i < N; i++)
if(l[i] + a[i] != r[i + 1] + a[i + 1])
ans = min(ans, max(maxL[i], maxR[i + 1]));
cout << ans << "\n";
return 0;
}
