#include <iostream>
#include <string>
#include <vector>
using namespace std;

int main() {


    int n;
    cin >> n;

    vector<int> A(n);
    for (int i = 0; i < n; i++) {
        cin>>A[i];
    }

    int odd_count = 0,even_count=0;
    for (int i = 0; i < n; ++i) {
        if (A[i] % 2 == 1) {
            odd_count++;
        }
        else
            even_count++;
    }

         if(odd_count==0){
            cout<<(n*(n+1))/2<<endl;

    }
        else if(even_count==0)
    {
            cout<<n-1<<endl;

    }
        else if(even_count!=0 && odd_count!=0) {

        cout << odd_count + 1 << endl;

    }
        else{
            cout<<-1<<endl;
        }

    return 0;
}