#include <iostream>
#include <queue>
#include <set>
using namespace std;

void ins() {}
template<class T,class... Rest>void ins(T& v,Rest&... rest){cin>>v;ins(rest...);}
#define rep(i,n) for(int i=0,_i=(n);i<_i;++i)

int main() {
  int N;
  ins(N);

  deque<int> A;
  rep(i, N) {
    int a; cin >> a;
    if (A.empty() || A.back() != a)
      A.push_back(a);
  }
  int ans = 0;
  if (A.size() == 1) {
    cout << 0 << endl;
    return 0;
  }
  if (A.front() == A.back()) {
    ans = 1;
    A.pop_back();
  }
  set<int> s;
  for (const auto& e:A) {
    if (s.find(e) != s.end()) {
      cout << -1 << endl;
      return 0;
    }
    s.insert(e);
  }
  cout << ans << endl;

  return 0;
}