#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for(int i = a; i < b; i++)
#define rrep(i, a, b) for(int i = a; i >= b; i--)
#define fore(i, a) for(auto &i:a)
typedef vector<int> vi;
typedef vector<vi> vvi;

int main() {
    int n, k, t, ans = 0;
    cin >> n >> k;
    vvi a(k);
    rep(i, 0, n) {
        cin >> t;
        a[i%k].push_back(t);
    }
    rep(i, 0, k) {
        rrep(j, a[i].size()-1, 1) {
            rep(l, 0, j) {
                if(a[i][l]>a[i][l+1]) {
                    swap(a[i][l], a[i][l+1]);
                    ans++;
                }
            }
        }
    }
    rep(i, 0, n-1) {
        t = i%k;
        if(t<k-1) {
            if(a[t][i/k]>a[t+1][i/k]) {
                ans = -1;
                break;
            }
        } else if(t==k-1) {
            if(a[t][i/k]>a[0][i/k+1]) {
                ans = -1;
                break;
            }
        }
    }
    printf("%d\n", ans);
}