#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cmath>
#include <map>
#include <queue>
#include <iomanip>
#include <set>
#include <tuple>
#define mkp make_pair
#define mkt make_tuple
#define rep(i,n) for(int i = 0; i < (n); ++i)
using namespace std;
typedef long long ll;
const ll MOD=1e9+7;
template<class T> void chmin(T &a,const T &b){if(a>b) a=b;}
template<class T> void chmax(T &a,const T &b){if(a<b) a=b;}

int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N;
  cin>>N;
  vector<int> A(N);
  rep(i,N) cin>>A[i];

  rep(i,N) A[i]--;

  vector<vector<int>> pos(N);
  rep(i,N){
    pos[A[i]].push_back(i);
  }

  int sum=0;
  for(int i=0;i<N;i++){
    if(pos[i].size()<=1) continue;
    int num=0;
    for(int j=0;j+1<pos[i].size();j++){
      if(pos[i][j]+1!=pos[i][j+1]){
        num++;
      }
    }
    sum+=num;
    if(num==0) continue;
    if(num>=2||A[0]!=i||A[N-1]!=i){
      cout<<-1<<endl;
      return 0;
    }
  }

  cout<<sum<<endl;

  return 0;
}