#include <bits/stdc++.h>

using namespace std;

#define fst(t) std::get<0>(t)
#define snd(t) std::get<1>(t)
#define thd(t) std::get<2>(t)
#define unless(p) if(!(p))
#define until(p) while(!(p))

using ll = long long;
using P = std::tuple<int,int>;

const int dx[8] = {-1, 1, 0, 0, -1, -1, 1, 1}, dy[8] = {0, 0, -1, 1, -1, 1, -1, 1};

int A[1010];

int main(){
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    int N, K;
    std::cin >> N >> K;

    for(int i=0;i<N;++i){
        std::cin >> A[i];
    }

    int cnt = 0;
    for(int i=0;i<K;++i){
        vector<int> v;
        for(int j=i;j<N;j+=K){
            v.emplace_back(A[j]);
        }

        int l = v.size();
        for(int i=0;i<l;++i){
            for(int j=0;j<i;++j){
                if(A[j] > A[i]){
                    ++cnt;
                }
            }
        }

        sort(v.begin(), v.end());

        for(int j=i;j<N;j+=K){
            A[j] = v[(j - i) / K];
        }
    }

    bool isSorted = true;
    for(int i=0;i+1<N;++i){
        if(A[i] > A[i + 1]){
            isSorted = false;
        }
    }

    if(isSorted){
        std::cout << cnt << std::endl;
    }else{
        std::cout << -1 << std::endl;
    }
}