#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <numeric>
#include <functional>
#include <cmath>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <sstream>
#include <string>
#define repd(i,a,b) for (int i=(int)(a);i<(int)(b);i++)
#define rep(i,n) repd(i,0,n)
#define all(x) (x).begin(),(x).end()
#define mod 1000000007
#define inf 2000000007
#define mp make_pair
#define pb push_back
typedef long long ll;
using namespace std;
template <typename T>
inline void output(T a, int p) {
    if(p) cout << fixed << setprecision(p)  << a << "\n";
    else cout << a << "\n";
}
// end of template

int main() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    // source code
    int N, K;
    cin >> N >> K;
    vector<int> A(N);
    rep(i, N){
        cin >> A[i];
    }
    
    int cnt = 0;
    
    rep(i, N) for(int j = i + K; j < N; j += K){
        if (A[i] > A[j]){
            swap(A[i], A[j]);
            cnt++;
        }
    }
    
    bool isSorted = true;
    repd(i, 1, N){
        if (A[i - 1] > A[i]) {
            isSorted = false;
            break;
        }
    }
    if (!isSorted){
        output(-1, 0);
    }
    else {
        output(cnt, 0);
    }
    
    
    
    return 0;
}