#include <bits/stdc++.h>
using namespace std;
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]--;
  }
  vector<int> bit(N + 1, 0);
  long long C = 0;
  for (int i = 0; i < N; i++){
    C += i;
    int p = P[i];
    while (p > 0){
      C -= bit[p];
      p -= p & -p;
    }
    p = P[i] + 1;
    while (p <= N){
      bit[p]++;
      p += p & -p;
    }
  }
  long long A = (C + M - 1) / M * M;
  if ((A - C) % 2 == 0){
    cout << A << endl;
  } else {
    if (M % 2 == 0){
      cout << -1 << endl;
    } else {
      cout << A + M << endl;
    }
  }
}