#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define rep(i,s,e) for(i64 (i) = (s);(i) < (e);(i)++)
#define all(x) x.begin(),x.end()

template<class T>
static inline std::vector<T> ndvec(size_t&& n, T val) noexcept {
  return std::vector<T>(n, std::forward<T>(val));
}

template<class... Tail>
static inline auto ndvec(size_t&& n, Tail&&... tail) noexcept {
  return std::vector<decltype(ndvec(std::forward<Tail>(tail)...))>(n, ndvec(std::forward<Tail>(tail)...));
}

template<class T, class Cond>
struct chain {
  Cond cond; chain(Cond cond) : cond(cond) {}
  bool operator()(T& a, const T& b) const {
    if(cond(a, b)) { a = b; return true; }
    return false;
  }
};
template<class T, class Cond>
chain<T, Cond> make_chain(Cond cond) { return chain<T, Cond>(cond); }

int main() {
  i64 N;
  cin >> N;
  vector<i64> A(N);
  rep(i,0,N) {
    cin >> A[i];
  }
  if(std::count(all(A), A[0]) == N) {
    cout << 0 << endl;
    return 0;
  }
  i64 cut = 0;
  rep(i,0,N) {
    if(A[i] != A[(i - 1 + N) % N]) {
      cut = i;
      break;
    }
  }
  std::vector<i64> B(N);
  rep(i,0,N) {
    B[i] = A[(cut + i) % N];
  }

  std::bitset<101010> phi;
  rep(i,0,N) {
    if(i + 1 == N || B[i] != B[i + 1]) {
      if(phi.test(B[i])) {
        cout << -1 << endl;
        return 0;
      }
      phi.set(B[i]);
    }
  }
  if(cut == 0) {
     cout << 0 << endl;
  }
  else {
    cout << 1 << endl;
  }
}