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

typedef long long ll;
typedef long double ld;
typedef vector < long long > vll;
typedef pair < long long, long long > pll;
typedef pair < int, int > pii;
typedef vector < int > vii;

#define csl ios_base::sync_with_stdio(false); cin.tie(NULL)
#define l(x) (((x) << 1) | 1)
#define r(x) ((l(x)) + 1)
#define mp make_pair
#define fst first
#define snd second

ll t, n, u, v, m, q, r, ql, qr, k, l, s, x, y, w;
const int N = 3e6 + 500;
const int shift = 1e6 + 50;
const long long mod = 1e9 + 7;
const long long INF = 1LL << 57LL;
const bool JUDGE = false;
int p[N / 2], revp[N / 2], cnt[N];
int main(){
    csl;
    if (JUDGE) {
        freopen("in.txt", "r", stdin);
        freopen("out.txt", "w", stdout);
    }
    cin >> n;
    cin >> p[1];
    revp[1] = p[1];
    for (int i = 0; i < n - 1; ++i) {
        cin >> p[i + 2];
        revp[n - i] = p[i + 2];
    }
    for (int i = 1; i <= n; ++i) {
        ll x = (i - p[i]) % n;
        ll y;
        y = (x > 0) ? x - n : n + x;
        if (abs(x) <= abs(y)) cnt[x + shift]++;
        else cnt[y + shift]++;
        //cout << x << ' ' << y << '\n';
    }
    ll ans = 0;
    ll ct = 0;
    ll jj = 0;
    bool yy = true;
    for (int i = (1 - ceil((double)n / 2.0)) + (int)shift; i <= floor((double)n / 2.0) + shift; ++i) {
        if (!cnt[i]) {
            ct++;
            if (yy) jj++;
        }
        else {
            if (yy) yy = false;
            ans = max(ans, ct);
            ct = 0;
            cnt[i] = 0;
            continue;
        }
    }
    ans = max(ans, ct + jj);
    ct = 0;
    for (int i = 1; i <= n; ++i) {
        ll x = (i - revp[i]) % n;
        ll y;
        y = (x > 0) ? x - n : n + x;
        if (abs(x) <= abs(y)) cnt[x + shift]++;
        else cnt[y + shift]++;
    }
    int j = 0;
    bool kx = true;
    for (int i = (1 - ceil((double)n / 2.0)) + (int)shift; i <= floor((double)n / 2.0) + shift; ++i) {
        if (!cnt[i]) {
            ct++;
            if (kx) j++;
        }
        else {
            if (kx) kx = false;
            ans = max(ans, ct);
            ct = 0;
            cnt[i] = 0;
            continue;
        }
    }
    ans = max(ans, ct + j);
    int i = n / 2;
    j = (int)abs(1 - ceil(double(n) / 2.0));
    if (i == j) cout << i - (ans / 2) << '\n';
    else cout << max(i, j) - ((ans + 1) / 2) << '\n';
    return 0;
}