#include <stdio.h>

void add_BIT(int N, int BIT[], int i, int k)
{
	while (i <= N) {
		BIT[i] += k;
		i += (i & -i);
	}
}

long long sum_BIT(int BIT[], int r)
{
	long long sum = 0;
	while (r > 0) {
		sum += BIT[r];
		r -= (r & -r);
	}
	return sum;
}

long long inversion_number(int N, int A[], int B[])
{
	int i, a[200001], BIT[200001] = {};
	long long inv = 0;
	for (i = 1; i <= N; i++) a[A[i]] = i;
	for (i = N; i >= 1; i--) {
		inv += sum_BIT(BIT, a[B[i]]);
		add_BIT(N, BIT, a[B[i]], 1);
	}
	return inv;
}

int main()
{
	int i, N, P[200001];
	long long M;
	scanf("%d %lld", &N, &M);
	for (i = 1; i <= N; i++) scanf("%d", &(P[i]));
	
	int I[200001];
	for (i = 1; i <= N; i++) I[i] = i;
	
	long long inv_num = inversion_number(N, P, I), ans;
	if (M % 2 == 0 && inv_num % 2 != 0) printf("-1\n");
	else {
		ans = (inv_num + M - 1) / M * M;
		if (ans % 2 != inv_num % 2) ans += M;
		printf("%lld\n", ans);
	}
	fflush(stdout);
	return 0;
}