#include <iostream>
#include <algorithm>
#include <utility>
#include <vector>
#include <numeric>

template <class T, class U>
inline bool chmin(T &lhs, const U &rhs) {
  if (lhs > rhs) {
    lhs = rhs;
    return true;
  }
  return false;
}

template <class T, class U>
inline bool chmax(T &lhs, const U &rhs) {
  if (lhs < rhs) {
    lhs = rhs;
    return true;
  }
  return false;
}

// [l, r) from l to r
struct range {
  struct itr {
    int i;
    constexpr itr(int i_): i(i_) { }
    constexpr void operator ++ () { ++i; }
    constexpr int operator * () const { return i; }
    constexpr bool operator != (itr x) const { return i != x.i; }
  };
  const itr l, r;
  constexpr range(int l_, int r_): l(l_), r(std::max(l_, r_)) { }
  constexpr itr begin() const { return l; }
  constexpr itr end() const { return r; }
};

// [l, r) from r to l
struct revrange {
  struct itr {
    int i;
    constexpr itr(int i_): i(i_) { }
    constexpr void operator ++ () { --i; }
    constexpr int operator * () const { return i; }
    constexpr bool operator != (itr x) const { return i != x.i; }
  };
  const itr l, r;
  constexpr revrange(int l_, int r_): l(l_ - 1), r(std::max(l_, r_) - 1) { }
  constexpr itr begin() const { return r; }
  constexpr itr end() const { return l; }
};

int main() {
  int N;
  std::cin >> N;
  std::vector<int> A(N);
  for (int &x: A) {
    std::cin >> x;
    --x;
  }
  std::vector<int> col(N, -1);
  int bad = 0;
  for (int i: range(0, N)) {
    if (col[A[i]] == -1 || col[A[i]] == i - 1) {
      col[A[i]] = i;
    }
    else {
      ++bad;
    }
  }
  if (bad == 0) {
    std::cout << "0\n";
  }
  else if (bad == 1 && A.front() == A.back()) {
    std::cout << "1\n";
  }
  else {
    std::cout << "-1\n";
  }
  return 0;
}