#include<bits/stdc++.h>
using namespace std;
int main(){
    int n;
    cin>>n;
    int a[n];
    vector<vector<int>>color;
    for(int i=0;i<n;i++){
        cin>>a[i];
        if(color.size()<a[i])color.resize(a[i]);
        color[a[i]-1].push_back(i);
    }
    int rev=0;
    for(int i=0;i<color.size();i++){
        for(int j=0;j<color[i].size()-1;j++){
            if(color[i][j]+1<color[i][j+1]){
                if(color[i][0]!=0 || color[i].back()!=n-1){
                    cout<<"-1"<<endl;
                    return 0;
                }
                rev++;
                for(int k=j+1;k<color[i].size()-1;k++){
                    if(color[i][k]+1<color[i][k+1]){
                        cout<<"-1"<<endl;
                        return 0;
                    }
                }
                break;
            }
        }
    }
    cout<<rev<<endl;
    return 0;
}