cf-1367-B

/*  BISMILLAHIR-RAHMANIR-RAHIM
 ____________________________________
|                                    |
|       SHANTO_SUST_SWE-19_029       |
|____________________________________|
*/

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
#include <time.h>

#define debug 1

int main()
{
    int t;
    scanf("%d", &t);
    int n;
    int i, even, odd, count;

    while(t--){
        scanf("%d", &n);
        int ara[n];

        even  = 0, odd = 0, count = 0;

        for(i = 0; i < n; i++){
            scanf("%d", &ara[i]);
            if(ara[i] % 2 == 0) even++;
            else odd++;
        }

        if(n % 2 == 0){
            if(even == odd){
                for(i = 0; i < n; i++){
                    if(i % 2 != ara[i] % 2) count++;
                }
                printf("%d\n", count / 2);
            }
            else printf("-1\n");
        }
        else{
            if(even == odd + 1){
                for(i = 0; i < n; i++){
                    if(i % 2 != ara[i] % 2) count++;
                }
                printf("%d\n", count / 2);
            }
            else printf("-1\n");
        }
    }


    return 0;
}

//ALHAMDULILLAH