#include<bits/stdc++.h>
using namespace std;
#define int long long
#define rep(i,n) for(int i=0;i<n;i++)
int a[100006];
signed main(){
    int n;cin>>n;
    rep(i,n){
        cin>>a[i];
        assert(a[i]<=n);
    }
    vector<int>v;
    int now=a[0];
    rep(i,n+1){
        if(now!=a[i]){
            v.push_back(now);
            now=a[i];
        }
    }
    set<int>S;
    int flg=0;
    for(int e:v){
        if(S.find(e)!=S.end())flg++;
        S.insert(e);
    }
    if(flg>1)flg=-1;
    cout<<flg<<endl;
}