#include <bits/stdc++.h>
using namespace std;

#include <atcoder/fenwicktree>
long long inversion_number(vector<int> &a) {
  int n=a.size();
  atcoder::fenwick_tree<int> ft(n);
  long long ret=0;
  for (int i=0;i<n;i++) {
    ret+=i-ft.sum(0,a[i]);
    ft.add(a[i],1);
  }
  return ret;
}

int main() {
  int n;
  long long m;
  cin>>n>>m;
  vector<int> p(n);
  for (int i=0;i<n;i++) {
    cin>>p[i];
    p[i]--;
  }
  long long in=inversion_number(p);
  if (in==0) {
    cout<<0<<endl;
    return 0;
  }
  else if (in%2==1&&m%2==0) {
    cout<<-1<<endl;
    return 0;
  }
  long long tmp=(in+m-1)/m*m;
  while ((tmp-in)%2!=0) {
    tmp+=m;
  }
  cout<<tmp<<endl;
}