#include<iostream>
#include<cassert>
#include<atcoder/fenwicktree>
using namespace std;
int main()
{
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int N;
	long M;
	cin>>N>>M;
	atcoder::fenwick_tree<int>BIT(N);
	long K=0;
	for(int i=0;i<N;i++)
	{
		int P;cin>>P;P--;
		K+=BIT.sum(P,N);
		BIT.add(P,1);
	}
	if(K==0)cout<<"0\n";
	else if(M%2==0)
	{
		if(K%2==1)
		{
			cout<<"-1\n";
		}
		else
		{
			if(M<=K)cout<<(K+M-1)/M*M<<"\n";
			else cout<<M<<"\n";
		}
	}
	else
	{
		if(M<=K)
		{
			long t=(K+M-1)/M*M;
			if(K%2!=t%2)t+=M;
			cout<<t<<"\n";
		}
		else if(K%2==M%2)cout<<M<<"\n";
		else cout<<M+M<<"\n";
	}
}