#ifdef LOCAL111
#else
	#define NDEBUG
#endif
#include <bits/stdc++.h>
const int INF = 1e8;
using namespace std;

#define endl '\n'
#define ALL(a)  (a).begin(),(a).end()
#define SZ(a) int((a).size())
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n)  FOR(i,0,n)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
#define RBP(i,a) for(auto& i : a)
#ifdef LOCAL111
	#define DEBUG(x) cout<<#x<<": "<<(x)<<endl
#else
	#define DEBUG(x) true
#endif
#define F first
#define S second
#define SNP string::npos
#define WRC(hoge) cout << "Case #" << (hoge)+1 << ": "
#define rangej(a,b,c) ((a) <= (c) and (c) < (b))
#define rrangej(b,c) rangej(0,b,c)
template<typename T> void pite(T a, T b){ for(T ite = a; ite != b; ite++) cout << (ite == a ? "" : " ") << *ite; cout << endl;}
template<typename T> bool chmax(T& a, T b){if(a < b){a = b; return true;} return false;}
template<typename T> bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;}

typedef pair<int,int> P;
typedef long long int LL;
typedef unsigned long long ULL;
typedef pair<LL,LL> LP;

void ios_init(){
	//cout.setf(ios::fixed);
	//cout.precision(12);
#ifdef LOCAL111
	return;
#endif
	ios::sync_with_stdio(false); cin.tie(0);	
}


int main()
{
	ios_init();
	int n,k;
	cin >> n >> k;
	vector<int> a(n);
	vector<int> af(n);
	REP(i,n) cin >> a[i];
	int ans = 0;
	REP(p,k){
		vector<P> b;
		for(int j = p; j < n; j += k){
			b.emplace_back(a[j],j/k);
		}
		REP(i,SZ(b)) FOR(j,i+1,SZ(b)){
			if(b[i] > b[j]) ans++;
		}
		sort(ALL(b));
		REP(i,SZ(b)){
			af[i*k+p] = b[i].F;
			//ans += abs(b[i].S-i);
		}
	}
	sort(ALL(a));
	if(a == af) cout << ans << endl;
	else cout << -1 << endl;
	return 0;
}