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

//template
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define ALL(v) (v).begin(),(v).end()
typedef long long int ll;
const int inf = 0x3fffffff; const ll INF = 0x1fffffffffffffff; const double eps=1e-12;
template<class T>inline bool chmax(T& a,T b){if(a<b){a=b;return 1;}return 0;}
template<class T>inline bool chmin(T& a,T b){if(a>b){a=b;return 1;}return 0;}
//end

bool ok(vector<int> v){
   set<int> used;
   rep(i,0,v.size()){
      if(used.count(v[i]) and v[i-1]!=v[i])return 0;
      used.insert(v[i]);
   } return 1;
}

bool pos(vector<int> v){
   deque<int> deq; int t=v.front();
   rep(i,0,v.size())deq.push_back(v[i]);
   while(deq.size()>1 and deq.front()==deq.back() and deq.front()==t){
      deq.pop_back(); deq.pop_front();
   } vector<int> tmp;
   for(int x:deq)tmp.push_back(x);
   return ok(tmp);
}

int main(){
   int n; scanf("%d",&n);
   vector<int> a(n),cmp;
   rep(i,0,n){
      scanf("%d",&a[i]);
      if(cmp.empty() or a[i]!=cmp.back())cmp.push_back(a[i]);
   }
   if(ok(cmp))puts("0");
   else if(pos(cmp))puts("1");
   else puts("-1");
   return 0;
}