#define _GLIBCXX_DEBUG
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define ALL(v) v.begin(), v.end()
typedef long long ll;

template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }

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


int main(){
  int n;
  cin>>n;
  
  vector<int> a(n),b(n),c(n);
  rep(i,n) cin>>a[i];
  rep(i,n) b[i]=a[i];
  rep(i,n) c[i]=a[i];
  
  c.erase(unique(ALL(c)),c.end());
  int z=c.size();
  
  sort(ALL(b));
  b.erase(unique(ALL(b)),b.end());
  int x=b.size();
  
  if(x==z) cout<<0<<endl;
  else{
    if(a[0]!=a[n-1]) cout<<-1<<endl;
    else{
      a.erase(unique(ALL(a)),a.end());
      int y=a.size();
      if(y==x+1) cout<<1<<endl;
      else cout<<-1<<endl;
    }
  }
  
  
  return 0;
}