#include<bits/stdc++.h>
using namespace std;
const int N = 1005;
int a[N] , c[N];
pair<int,pair<int,int>> b[N];
bool vis[N];
int main(){
    cin.tie(0)->sync_with_stdio(0);
    int n , k;
    memset(vis,false,sizeof(vis));
    bool kt = true;
    cin >> n >> k;
    for(int i = 1;  i <= n ; i ++){
        cin >> a[i];
        b[i].first = a[i];
        b[i].second.first = i;
    }
    sort(b + 1, b + n + 1);
    int x = 0;
    for(int i = 1 ; i <= n ; i ++){
        b[i].second.second = i;
    }
    for(int i = 1; i <= n ;i ++){
        if(abs(b[i].second.second - b[i].second.first) % k != 0){
            cout << -1;
            return 0;
        }
    }
    for(int i = 1; i <= n; i++){
        c[b[i].second.first] = b[i].second.second;
    }
    int ans = 0;
    for(int i = 1; i <= n; i++){
        if(vis[i] || c[i] == i) continue;
        int cur = i, len = 0;
        while(!vis[cur]){
            vis[cur] = true;
            cur = c[cur];
            len++;
        }
        ans += len - 1;
    }
    cout << ans;
}