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

int main() {
  ios::sync_with_stdio(false);

  int N; { cin >> N; }
  vector<int> A(N); { for (int i = 0; i < N; ++i) cin >> A[i]; }

  int res = 0;
  A.erase(unique(A.begin(), A.end()), A.end());
  if (A.back() == A.front()) ++res, A.pop_back();

  set<int> bag(A.begin(), A.end());
  if (bag.size() == A.size()) cout << res << endl;
  else cout << -1 << endl;
}