#include <iostream>
#include <vector>
#include <string>
#include <cstring>
#include <math.h>
#include <cmath>
#include <limits.h>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <functional>
#include <stdio.h>
using namespace std;

long long MOD = 1000000007;

int func( vector<int> v ) {
    int ret = 0;
    for ( int i = 0; i < v.size(); i++ ) {
    for ( int j = i+1; j < v.size(); j++ ) {
        if ( v[i] > v[j] ) { ret++; }
    }
    }
    return ret;
}

int main() {
    int N,K;
    cin >> N >> K;
    vector<int> V(N), VS(N);
    for ( int i = 0; i < N; i++ ) {
        cin >> V[i];
        VS[i] = V[i];
    }
    int a = 0;
    for ( int i = 0; i < K; i++ ) {
        vector<int> T;
        for ( int j = i; j < N; j+=K ) {
            T.push_back(V[j]);
        }
        a += func(T);

        sort( T.begin(), T.end() );
        for ( int j = i; j < N; j+=K ) {
            V[j] = T[(j-i)/K];
        }
    }

    sort( VS.begin(), VS.end() );
    for ( int i = 0; i < N; i++ ) {
        if ( V[i] != VS[i] ) { a = -1; }
    }
    cout << a << endl;

    return 0;
}