#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void umax(T &a, T b) {
  if (a < b) a = b;
}
template <class T>
inline void umin(T &a, T b) {
  if (a > b) a = b;
}
template <class T>
inline T abs(T a) {
  return a > 0 ? a : -a;
}
template <class T>
inline T gcd(T a, T b) {
  return __gcd(a, b);
}
template <class T>
inline T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
long long modpow(long long a, long long n, long long temp) {
  long long res = 1, y = a;
  while (n > 0) {
    if (n & 1) res = (res * y) % temp;
    y = (y * y) % temp;
    n /= 2;
  }
  return res % temp;
}
long long p2(long long n) { return (1ll << n); }
long long getbit(long long j, long long i) { return ((i >> (j - 1ll)) & 1ll); }
long long ison(long long mask, long long pos) { return (mask & (1 << pos)); }
long long cbit(long long n) {
  long long k = 0;
  while (n) n &= (n - 1), k++;
  return k;
}
long long nbit(long long n) {
  long long k = 0;
  while (n) n /= 2, k++;
  return k;
}
long long mod = 998244353;
int sgn(long long x) { return x < 0 ? -1 : !!x; }
const double eps = 6 * 1e-8;
const double pi = acos(-1.0);
inline void inc(long long &x, long long y) {
  x += y;
  if (x >= mod) x -= mod;
}
inline void dec(long long &x, long long y) {
  x -= y;
  if (x < 0) x += mod;
}
inline void chmax(long long &x, long long y) {
  if (y > x) x = y;
}
inline void mulm(long long &x, long long y) {
  x *= y;
  if (x >= mod) x -= mod;
}
long long xo(long long i) {
  if ((i & 3) == 0) return i;
  if ((i & 3) == 1) return 1;
  if ((i & 3) == 2) return i + 1;
  return 0;
}
long long a[200005];
int main() {
  long long n, m, j, k, l;
  long long ans = mod;
  cin >> n;
  if (n <= 2) {
    cout << 0;
    return 0;
  }
  for (long long i = 1; i <= n; i++) cin >> a[i];
  for (long long i = -1; i <= 1; i++) {
    for (long long j = -1; j <= 1; j++) {
      long long u = (a[n] + j - (a[1] + i));
      if (u % (n - 1ll) == 0) {
        long long ch = (i != 0);
        u /= (n - 1ll);
        long long a1 = a[1] + i;
        for (long long k = 2; k <= n; k++) {
          long long x = (a1 + (k - 1) * u);
          if (abs(x - a[k]) > 1)
            ch = mod;
          else
            ch += (abs(x - a[k]));
        }
        ans = min(ans, ch);
      }
    }
  }
  if (ans == mod)
    cout << -1;
  else
    cout << ans;
}
