Two.java

// Twitter Codibility Screening

public class Two {

    public int solution(int[] A) {
        int evens = getEvensCount(A);
        long totalPairs = nChooseK(evens, 2) + nChooseK(A.length - evens, 2);
        return totalPairs > 1000000000 ? -1 : (int) totalPairs;
    }

    private int getEvensCount(int[] A) {
        int count = 0;
        for (int i = 0; i < A.length; i++) {
            count += (A[i] % 2) == 0 ? 1 : 0;
        }
        return count;
    }

    private long nChooseK(long n, long k) {
        return k == 0 ? 1: ((n * nChooseK(n - 1, k - 1)) / k);
    }
    
}