#include <bits/stdc++.h>
#define REP(i,n) for(int i = 0; i < (int)(n); ++i)
using namespace std;

int main(){
    int N, K, t;
    cin >> N >> K;
    vector<int> A, B;
    REP(i,N){
        cin >> t;
        A.push_back(t);
        B.push_back(t);
    }
    sort(B.begin(),B.end());

    int c = 0;
    int flag = 1;
    while(flag){
        flag = 0;
        for(int j = N-1; j >= K; j--){
            if(A[j] < A[j-K]){
                swap(A[j],A[j-K]);
                c++;
                flag = 1;
            }
        }
    }
    bool b = true;
    REP(i,N){
        if(A[i] != B[i]) b = false;
    }
    cout << (b?c:-1) << endl;
    return 0;
}