#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    cin>>n;
    int a[n];
    vector<int>color;
    for(int i=0;i<n;i++){
        cin>>a[i];
        if(i==0)color.push_back(a[i]);
        if(i && a[i]!=a[i-1])color.push_back(a[i]);
    }
    int x[n]={};
    int b=0;
    for(int i=0;i<color.size();i++){
        x[color[i]-1]++;
        if(x[color[i]-1]==2){
            b=1;
            if(a[0]!=color[i]||a[n-1]!=color[i]){
                cout<<"-1"<<endl;
                return 0;
            }
        }
        if(x[color[i]-1]==3){
            cout<<"-1"<<endl;
            return 0;
        }
    }
    cout<<b<<endl;
    return 0;
}