#include <bits/stdc++.h>
using namespace std;

typedef long long   signed int LL;
typedef long long unsigned int LU;

#define incID(i, l, r) for(int i = (l)    ; i <  (r); i++)
#define incII(i, l, r) for(int i = (l)    ; i <= (r); i++)
#define decID(i, l, r) for(int i = (r) - 1; i >= (l); i--)
#define decII(i, l, r) for(int i = (r)    ; i >= (l); i--)
#define  inc(i, n) incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define  dec(i, n) decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)

#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define inID(v, l, r) ((l) <= (v) && (v) <  (r))

#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define UB upper_bound
#define LB lower_bound
#define PQ priority_queue

#define  ALL(v)  v.begin(),  v.end()
#define RALL(v) v.rbegin(), v.rend()
#define  FOR(it, v) for(auto it =  v.begin(); it !=  v.end(); ++it)
#define RFOR(it, v) for(auto it = v.rbegin(); it != v.rend(); ++it)

template<typename T> bool   setmin(T & a, T b) { if(b <  a) { a = b; return true; } else { return false; } }
template<typename T> bool   setmax(T & a, T b) { if(b >  a) { a = b; return true; } else { return false; } }
template<typename T> bool setmineq(T & a, T b) { if(b <= a) { a = b; return true; } else { return false; } }
template<typename T> bool setmaxeq(T & a, T b) { if(b >= a) { a = b; return true; } else { return false; } }
template<typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template<typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

// ---- ----

LL n, a[2000];
LL dp[2][2000];

int main() {
	cin >> n;
	inc(i, n) { cin >> a[i]; }
	
	LL l = a[0], r = a[n - 1];
	inc(i, n) { setmax(l, a[i] - i);          dp[0][i] = l; }
	dec(i, n) { setmax(r, a[i]- (n - 1 - i)); dp[1][i] = r; }
	
	
	inc(i, n) {
		cout << dp[0][i] << " " << dp[1][i] << endl;
	}
	
	LL ans = 1000002000;
	inc(i, n) { setmin(ans, max(dp[0][i] + n - 1, dp[1][i + 1]) + n - 1 - (i + 1)); }
	inc(i, n) { setmin(ans, max(dp[1][i + 1] + n - 1, dp[0][i]) + i); }
	
	cout << ans << endl;
	
	return 0;
}