CF_B. Even Array.cpp

#include<bits/stdc++.h>
using namespace std;


int main()
{
    int t;
    cin>>t;
    while(t--)
    {
        int n;
        cin>>n;
        int a=0,b=0;
        
        for(int i=0;i<n;i++)
        {
            int d;
            cin>>d;
            if(d%2 != i%2)
            {
                if(i%2 == 0)
                {
                    a++;
                }
                else{
                    b++;
                }
             
            }
            
        }
        if(a!=b)
        {
            cout<<-1<<"\n";
        }
        else{
            cout<<a<<"\n";
        }
        
    }
    return 0;
}