#include<bits/stdc++.h>
#define pll pair<ll, ll>
#define fi first
#define se second
#define pb push_back
#define pii pair<ll, pll>
using namespace std;
using ll = int;
const int N = 2e5+55;
const ll mod = 1e9+7;
const ll mod1 = 1e9+1;
const ll base = 311;
const ll base1 = 331;
ll m, n, t, k, a[N], tong, b[N], dp[N], c[N], lab[N], l[N], r[N], cnt, ans;
string s[N];
vector<pll> adj[N];
vector<pll> e;
ll pw(ll k, ll n)
{
ll total = 1;
for(; n; n >>= 1)
{
if(n & 1)total = total * k % mod;
k = k * k % mod;
}
return total;
}
bool check(ll x)
{
ll lf = n, rt = 1;
l[1] = 0;
for(int i = 2; i <= n; i ++)
{
l[i] = max(l[i-1], a[i-1] + l[i-1] + 1 - a[i]);
if(l[i] > x)
{
lf = i - 1;
break;
}
}
r[n] = 0;
for(int i = n-1; i > 0; i --)
{
r[i] = max(r[i+1], a[i+1] + r[i+1] + 1 - a[i]);
if(r[i] > m)
{
rt = i + 1;
break;
}
}
return lf >= rt;
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n;
for(int i = 1; i <= n; i ++)
{
cin >> a[i];
}
ll lf = 0, rt = mod, mid;
while(lf <= rt)
{
mid = (lf + rt) / 2;
if(check(mid))rt = mid - 1;
else lf = mid + 1;
}
cout << lf;
}
/*
1
5 9
1 2 3 2
1 3 2 -2
2 2 3
1 3 2 -3
2 2 3
1 1 4 7
1 4 5 8
2 5 2
2 5 1
1 5 6
1 1 2 2
1 2 4 3
2 1 4
1 1 5 6
1 2 3 -2
2 3 5
*/
