#include<bits/stdc++.h>

using namespace std;

int main() {
    int n;
    cin>>n;
    int a[n];
    for (int i = 0; i < n; ++i) {
        cin>>a[i];
    }
    int count=0;
    int sum=0;
    for (int i = 0; i < n; ++i) {
        sum+=a[i];
        if (sum%2==0){
            count++;
            sum=0;
        }
    }
    if (count==0) cout<< -1<<endl;
    else
    cout<<count;
    return 0;
}