#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long ll;
typedef long double ld;
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPR(i, n) for (int i = n - 1; i >= 0; --i)
#define FOR(i, m, n) for (int i = m; i < n; ++i)
#define FORR(i, m, n) for (int i = m; i >= n; --i)
#define ALL(v) (v).begin(),(v).end()
#define ALLR(v) (v).rbegin(),(v).rend()
#define fi first
#define se second
#define PB push_back
#define EB emplace_back
template<typename T> using PQ = priority_queue<T>;
template<typename T> using QP = priority_queue<T,vector<T>,greater<T>>;
template<typename T>void debug(const T &v,ll h,ll w){for(ll i=0;i<h;i++){for(ll j=0;j<w;j++)cerr<<v[i][j]<<" \n"[j==w-1];}};
template<typename T>void debug(const T &v,ll n){for(ll i=0;i<n;i++)cerr<<v[i]<<" \n"[i==n-1];};
template<typename T>void debug(const vector<T>&v){debug(v,v.size());}
template<typename T>void debug(const vector<vector<T>>&v){for(auto &vv:v)debug(vv,vv.size());}
template<typename T>void debug(stack<T> st){while(!st.empty()){cerr<<st.top()<<" ";st.pop();}cerr<<endl;}
template<typename T>void debug(queue<T> st){while(!st.empty()){cerr<<st.front()<<" ";st.pop();}cerr<<endl;}
template<typename T>void debug(deque<T> st){while(!st.empty()){cerr<<st.front()<<" ";st.pop_front();}cerr<<endl;}
template<typename T>void debug(PQ<T> st){while(!st.empty()){cerr<<st.top()<<" ";st.pop();}cerr<<endl;}
template<typename T>void debug(QP<T> st){while(!st.empty()){cerr<<st.top()<<" ";st.pop();}cerr<<endl;}
template<typename T>void debug(const set<T>&v){for(auto z:v)cerr<<z<<" ";cerr<<endl;}
template<typename T>void debug(const multiset<T>&v){for(auto z:v)cerr<<z<<" ";cerr<<endl;}
template<typename T,size_t size>void debug(const array<T, size> &a){for(auto z:a)cerr<<z<<" ";cerr<<endl;}
template<typename T,typename V>void debug(const map<T,V>&v){for(auto z:v)cerr<<"["<<z.first<<"]="<<z.second<<",";cerr<<endl;}
void foo() {cerr << endl;}
template <class Head, class... Tail>void foo(Head&& head, Tail&&... tail){cerr << head << " ";foo(move(tail)...);}
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
int popcount(ll x){return __builtin_popcountll(x);};
int poplow(ll x){return __builtin_ctzll(x);};
int pophigh(ll x){return 63 - __builtin_clzll(x);};
const ll INF=1LL<<60;
const int inf=(1<<30)-1;
// const int mod=1e9+7;
const int mod=998244353;
int dx[8]={1,0,-1,0,-1,-1,1,1};
int dy[8]={0,1,0,-1,-1,1,-1,1};

using mint=modint998244353;
// using mint=modint1000000007;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n,k;cin >> n >> k;
    vector<int> a(n);
    REP(i,n) cin >> a[i];
    int ans=0;
    REP(i,k){
        vector<int> d;
        for(int j=i;j<n;j+=k) d.PB(a[j]);
        REP(s,d.size()) FOR(t,s+1,d.size()) if(d[s]>d[t]) ans++;
        sort(ALL(d));
        REP(s,d.size()) a[i+k*s]=d[s];
    }
    if(!is_sorted(ALL(a))) cout << -1 << endl;
    else cout << ans << endl;
}