Day75

//B. Even Array
//https://codeforces.com/problemset/problem/1367/B
import java.util.Scanner;
 
public class EvenArray {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        int t = scanner.nextInt(); 
        for (int i = 0; i < t; i++) {
            int n = scanner.nextInt(); 
            int[] a = new int[n];
            for (int j = 0; j < n; j++) {
                a[j] = scanner.nextInt(); 
            }
            
            int evenMismatch = 0; 
            int oddMismatch = 0;   
            
            // Count mismatches
            for (int j = 0; j < n; j++) {
                if (j % 2 == 0 && a[j] % 2 != 0) {
                    evenMismatch++; 
                } else if (j % 2 != 0 && a[j] % 2 == 0) {
                    oddMismatch++; 
                }
            }
            
           
            if (evenMismatch != oddMismatch) {
                System.out.println(-1);
            } else {
                System.out.println(evenMismatch); 
            }
        }
        
        scanner.close();
    }
}
//
//A. Panoramix's Prediction
//https://codeforces.com/problemset/problem/80/A

import java.util.Scanner;
 
public class PanoramixPrediction {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        // List of all prime numbers <= 50
        int[] primes = {2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47};
 
        
        int n = scanner.nextInt(); 
        int m = scanner.nextInt(); 
        
        // Find if m is the next prime after n
        boolean isNextPrime = false;
        for (int i = 0; i < primes.length - 1; i++) {
            if (primes[i] == n && primes[i + 1] == m) {
                isNextPrime = true;
                break;
            }
        }
 
   
        System.out.println(isNextPrime ? "YES" : "NO");
        
        scanner.close();
    }
}
//A. Boring Apartments
//https://codeforces.com/problemset/problem/1433/A
import java.util.Scanner;

public class BoringApartments {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        int t = scanner.nextInt(); // Number of test cases
        for (int i = 0; i < t; i++) {
            int x = scanner.nextInt(); // Apartment number that answered
            int digit = x / 1111; // Extracting the digit (1-9)
            int totalKeypresses = 0;

            // Count keypresses for digits 1 to (digit - 1)
            for (int d = 1; d < digit; d++) {
                totalKeypresses += d * 4; // Each digit contributes d keypresses for 4 apartments (d, dd, ddd, dddd)
            }

            // Count keypresses for the current digit
            totalKeypresses += digit * (x % 1111 + 1); // Count apartments from d, dd, ..., up to x

            System.out.println(totalKeypresses);
        }
        
        scanner.close();
    }
}

//202. Happy Number
//https://leetcode.com/problems/happy-number/description/?envType=study-plan-v2&envId=top-interview-150

class Solution {
    public boolean isHappy(int n) {
        HashSet<Integer> seen = new HashSet<>();
        
        while (n != 1 && !seen.contains(n)) {
            seen.add(n);
            n = getNext(n);
        }
        
        return n == 1;
    }

    private int getNext(int n) {
        int totalSum = 0;
        while (n > 0) {
            int digit = n % 10;
            totalSum += digit * digit;
            n /= 10;
        }
        return totalSum;
    }
}