#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <climits>
#include <iostream>
#include <vector>
#include <string>
#include <sstream>
#include <algorithm>
#include <utility>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>
#include <functional>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef pair<int, int> pii;
typedef vector<pii> vpii;

#define fst first
#define scd second
#define PB push_back
#define MP make_pair
#define rep(i,x) for(int i=0;i<(x);++i)
#define rep1(i,x) for(int i=1;i<=(x);++i)
#define rrep(i,x) for(int i=(x)-1;i>=0;--i)
#define rrep1(i,x) for(int i=(x);i>=1;--i)
#define FOR(i,a,x) for(int i=(a);i<(x);++i)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()

#define omajinai ios::sync_with_stdio(false);cin.tie(0)

template<typename T>bool chmax(T&a,T b){if(a<b){a=b;return true;}return false;}
template<typename T>bool chmin(T&a,T b){if(a>b){a=b;return true;}return false;}
template<typename T>T get(){T a;cin>>a;return a;}
template<typename T>T rev(T a){reverse(all(a));return a;}
template<typename T>istream&operator>>(istream&is,vector<T>&vec){rep(i,vec.size())is>>vec[i];return is;}
template<typename T>vector<T>&sort(vector<T>&a){sort(all(a));return a;}

const int inf = 1e9;
const ll linf = 3e18;
const double eps = 1e-9;

int N, K;

signed main()
{
    cin >> N >> K;

    vi A(N), rnk(N);

    rep(i, N) cin >> A[i];

    rep(i, N) {
        int cnt = 0;
        rep(j, N) {
            if (A[i] > A[j]) cnt++;
        }
        rnk[i] = cnt;
    }

    int ans = 0;

    rep(now, N) {
        rep(j, N) {
            if (rnk[j] != now) continue;

            int to = rnk[j];

            if (j == to) break;
            if (abs(to - j) % K) {
                cout << -1 << endl;
                return 0;
            }

            int v = j < to ? 1 : -1;

            int k = j;
            while (k != to) {
                ans++;
                swap(rnk[k], rnk[k + K * v]);
                k += K * v;
            }
        }
    }

    cout << ans << endl;
}