#include <bits/stdc++.h>

#define _overload(_1,_2,_3,name,...) name
#define _rep(i,n) _range(i,0,n)
#define _range(i,a,b) for(int i=int(a);i<int(b);++i)
#define rep(...) _overload(__VA_ARGS__,_range,_rep,)(__VA_ARGS__)

#define _rrep(i,n) _rrange(i,n,0)
#define _rrange(i,a,b) for(int i=int(a)-1;i>=int(b);--i)
#define rrep(...) _overload(__VA_ARGS__,_rrange,_rrep,)(__VA_ARGS__)

#define _all(arg) begin(arg),end(arg)
#define uniq(arg) sort(_all(arg)),(arg).erase(unique(_all(arg)),end(arg))
#define getidx(ary,key) lower_bound(_all(ary),key)-begin(ary)
#define clr(a,b) memset((a),(b),sizeof(a))
#define bit(n) (1LL<<(n))
#define popcount(n) (__builtin_popcountll(n))

template<class T>bool chmax(T &a, const T &b) { return (a<b)?(a=b,1):0;}
template<class T>bool chmin(T &a, const T &b) { return (b<a)?(a=b,1):0;}

using namespace std;

const int dx[8]={1,0,-1,0,1,-1,-1,1};
const int dy[8]={0,1,0,-1,1,1,-1,-1};

int a[1010],sa[1010];

int main(void){
	int n,k;
	cin >> n >> k;	
	rep(i,n) cin >> a[i],sa[i]=a[i];
	sort(sa,sa+n);

	int ans=0;
	rep(offset,k){
		int start=offset;
		while(1){
			if(start>=n) break;
			int cmax=1<<30,idx=-1;
			for(int i=start;i<n;i+=k) if(chmin(cmax,a[i])) idx=i;
			while(idx-k>=start) swap(a[idx],a[idx-k]),idx-=k,ans++;
			start+=k;
		}
	}

	bool ok=true;
	rep(i,n) if(a[i]!=sa[i]) ok=false;

	if(ok==false)
		cout << -1 << endl;
	else
		cout << ans << endl;
	return 0;
}