#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 even=0,odd=0;
    for (int num : A) {
        if (num%2==0) {
            even++;
        } else {
            odd++;
        }
    }

    if (even%2==1) {
        cout<<-1<<endl;
    } else {
        cout<<odd+1<<endl;
    }


    return 0;
}