# include <iostream>
# include <cstdio>

using namespace std;

const int Maxn = 130012;

int n, ans1, ans2, L1, L2, x, p[Maxn], q1, q2;

int GCD (int a, int b){

 if (b > a) return GCD(b, a);
 else
 if (b == 0) return a;
 else
return GCD(b, a % b);

}



int main () {
    freopen ("1.txt", "r", stdin);
    freopen ("2.txt", "w", stdout);

    cin>>n;

    for (int i = 1; i <= n; i++){
     cin>>x;
     p[x] = i;
    }

    q1 = 0;
    q2 = n + 1;
    ans1 = 0;
    ans2 = 0;

    for (int i = 1; i <= Maxn; i++){
        if (p[i] > 0) {
          q1 ++; q2 --;

          L1 = p[i] - q1; if (L1 < 0) L1 = -L1;
          L2 = p[i] - q2; if (L2 < 0) L2 = -L2;

          if (L1 > 0)
              if (ans1 > 0) ans1 = GCD(ans1, L1); else ans1 = L1;

          if (L2 > 0)
              if (ans2 > 0) ans2 = GCD(ans2, L2); else ans2 = L2;



        }

    }

    if (ans1 == 0) ans1 = n - 1; else ans1 --;
    if (ans2 == 0) ans2 = n - 1; else ans2 --;

    if (ans1 > ans2) cout<<ans1; else cout<<ans2;

return 0;
}

Edited by author 19.09.2012 17:23