#include <iostream>
#include <vector>
#include <set>
using namespace std;
int main(){
  int n;cin>>n;
  vector<int> A(n);
  for(int i = 0; n > i; i++){
    cin>>A[i];
  }
  int begi = 0;
  int eni = n-1;
  if(A[0] == A[n-1]){
    while(begi != n && A[0] == A[begi])begi++;
    if(begi == n){
      cout << 0 << endl;
      return 0;
    }
    while(A[n-1] == A[eni])eni--;
  }
  set<int> X;
  X.insert(A[begi]);
  for(int i = begi+1; eni >= i; i++){
    if(A[i] == A[0]){
      cout << -1 << endl;
      return 0;
    }
    if(A[i-1] != A[i]){
      if(X.count(A[i])){
        cout << -1 << endl;
        return 0;
      }
      X.insert(A[i]);
    }
  }
  if(begi == 0){
    cout << 0 << endl;
  }else{
    cout << 1 << endl;
  }
}