#include<bits/stdc++.h>

using namespace std;

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