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

struct thue
{
    ll c, t, p;
};

struct node
{
    ll nt;
    vector<ll> ds;
};

ll nghichthe = 0;



ll bbs(vector<ll> &list)
{
    ll count = 0;
    
    for(ll i=0;i<list.size()-1;i++)
    {
        bool sw = false;
        for(ll j=0;j<list.size()-i-1;j++)
        {
            if(list[j] > list[j+1]) 
            {
                swap(list[j], list[j+1]);
                count++;
                sw = true;
            }
        }
        if(!sw) break;
    }
    return count;

}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    ll n, k;
    cin >> n >> k;
    vector<ll> a(n);
    for(ll i=0;i<n;i++) cin >> a[i];
    unordered_map<ll, vector<ll>> cnt;
    ll ans = 0;
    for(ll i=0;i<n;i++)
    {
        cnt[(i+1)%k].push_back(a[i]);
    }
    sort(a.begin(), a.end());
    for(auto &d : cnt)
    {
        ans+=bbs(d.second);
        ll id = d.first;
        if(id==0) id = k;
        for(ll i=0;i<d.second.size();i++)
        {
            if(d.second[i] != a[id-1])
            {
                cout << -1;
                return 0;
            }
            id+=k;
        }
    }
    cout << ans;
    return 0;
}