#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001

long long get(vector<int> p){
	int n = p.size();
	fenwick_tree<long long> F(n);
	long long ret = 0;
	rep(i,n){
		ret += F.sum(p[i],n);
		F.add(p[i],1);
	}
	return ret;
}

int main(){
	
	int n;
	long long m;
	cin>>n>>m;
	vector<int> p(n);
	rep(i,n){
		cin>>p[i];
		p[i]--;
	}
	
	long long ret = get(p);
	
	long long ans = (ret+m-1)/m;
	ans *= m;
	if(ans%2!=ret%2)ans += m;
	if(ans%2!=ret%2){
		cout<<-1<<endl;
	}
	else cout<<ans<<endl;
	
	return 0;
}