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

long long inversion_number(vector<int> A){
    int N = A.size();
    vector<int> B = A;
    sort(B.begin(), B.end());
    map<int, int> mp;
    for (int i = 0; i < N; i++) mp[B[i]] = i + 1;
    for (int i = 0; i < N; i++) A[i] = mp[A[i]];
    long long ans = 0;
    vector<int> BIT(N + 1, 0);
    for (int i = 0; i < N; i++){
        ans += i; int j;
        j = A[i]; while (j > 0){ ans -= BIT[j]; j -= j & -j; }
        j = A[i]; while (j <= N){ BIT[j]++; j += j & -j; }
    }
    return ans;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int N,K; cin >> N >> K;
    vector<vector<int>> A(K);
    rep(i,N) {
        int a; cin >> a;
        A[i % K].push_back(a);
    }
    int ans = 0;
    rep(i,K) {
        ans += inversion_number(A[i]);
        sort(A[i].begin(), A[i].end());
        reverse(A[i].begin(), A[i].end());
    }
    vector<int> B;
    rep(i,N) {
        B.push_back(A[i % K].back());
        A[i % K].pop_back();
    }

    vector<int> C = B;
    sort(C.begin(), C.end());
    cout << (B == C ? ans : -1) << endl;
}