#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
//#include<cctype>
#include<climits>
#include<iostream>
#include<string>
#include<vector>
#include<map>
//#include<list>
#include<queue>
#include<deque>
#include<algorithm>
//#include<numeric>
#include<utility>
#include<complex>
//#include<memory>
#include<functional>
#include<cassert>
#include<set>
#include<stack>
#include<random>

const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef pair<int, int> pii;

const int MAXN = 1010;
int A[MAXN];
int B[MAXN];
int N, K;

int solve() {
    int ans = 0;
    for (int k = 0; k < K; k++) {
        vi C;
        for (int j = k; j < N; j+=K) C.push_back(A[j]);
        int sz = C.size();
        for (int i = 0; i < sz; i++) for (int j = i+1; j < sz; j++) {
            if (C[i] > C[j]) ans++;
        }
        sort(C.begin(), C.end());
        int it = 0;
        for (int j = k; j < N; j+=K) B[j] = C[it++];
    }
    for (int i = 0; i < N-1; i++) if (B[i] > B[i+1]) return -1;
    return ans;
}

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> N >> K;
    for (int i = 0; i < N; i++) cin >> A[i];
    cout << solve() << endl;
    return 0;
}