#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#undef _P
#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define ITR(x,c) for(__typeof(c.begin()) x=c.begin();x!=c.end();x++)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
//-------------------------------------------------------

int N,K;
int A[1010];
vector<int> V[1010];

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N>>K;
	FOR(i,N) cin>>x, V[i%K].push_back(x);
	int tot=0;
	FOR(i,K) {
		FOR(y,V[i].size()) FOR(x,y) if(V[i][x]>V[i][y]) tot++;
		sort(ALL(V[i]));
		FOR(x,V[i].size()) A[i+x*K] = V[i][x];
	}
	FOR(i,N-1) if(A[i]>A[i+1]) return _P("-1\n");
	_P("%d\n",tot);
	
	
	
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false);
	FOR(i,argc-1) s+=argv[i+1],s+='\n';
	FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	solve(); return 0;
}