#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using ll = long long;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    ll m, cnt = 0, cnt2;
    int n, v;
    cin >> n >> m;
    atcoder::fenwick_tree<int> fw(n);
    for(int i = 0; i < n; i++){
        cin >> v;
        cnt += fw.sum(v--, n);
        fw.add(v, 1);
    }
    cnt2 = (cnt + m - 1) / m * m;
    if((cnt2 - cnt) % 2 == 0){
        cout << cnt2 << '\n';
        return 0;
    }
    cnt2 += m;
    cout << (cnt2 - cnt & 1 ? -1 : cnt2) << '\n';
}