#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#define rep(i,n) for(int (i)=0;(i)<(int)(n);++(i))
#define rer(i,l,u) for(int (i)=(int)(l);(i)<=(int)(u);++(i))
#define reu(i,l,u) for(int (i)=(int)(l);(i)<(int)(u);++(i))
#if defined(_MSC_VER) || __cplusplus > 199711L
#define aut(r,v) auto r = (v)
#else
#define aut(r,v) __typeof(v) r = (v)
#endif
#define each(it,o) for(aut(it, (o).begin()); it != (o).end(); ++ it)
#define all(o) (o).begin(), (o).end()
#define pb(x) push_back(x)
#define mp(x,y) make_pair((x),(y))
#define mset(m,v) memset(m,v,sizeof(m))
#define INF 0x3f3f3f3f
#define INFL 0x3f3f3f3f3f3f3f3fLL
using namespace std;
typedef vector<int> vi; typedef pair<int, int> pii; typedef vector<pair<int, int> > vpii; typedef long long ll;
template<typename T, typename U> inline void amin(T &x, U y) { if(y < x) x = y; }
template<typename T, typename U> inline void amax(T &x, U y) { if(x < y) x = y; }

int main() {
	int N; int K;
	while(~scanf("%d%d", &N, &K)) {
		vector<int> a(N);
		for(int i = 0; i < N; ++ i)
			scanf("%d", &a[i]);
		int ans = 0;
		vi v;
		vi b(N);
		rep(k, K) {
			v.clear();
			for(int i = k; i < N; i += K)
				v.push_back(a[i]);
			rep(i, v.size()) rep(j, i)
				ans += v[i] < v[j];
			sort(all(v));
			for(int i = k, j = 0; i < N; i += K)
				b[i] = v[j ++];
		}
		if(!is_sorted(all(b)))
			puts("-1");
		else
			printf("%d\n", ans);
	}
	return 0;
}