#include<bits/stdc++.h>
using namespace std;
int r(){
int x=0,f=1;char ch=getchar();
while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
while(ch>='0'&&ch<='9'){x=x*10+ch-48;ch=getchar();}
return x*f;
}
int n=r(),k=r(),a,t2[101022];string s,s2,s3,t1[101022];map<string,bool>sb;
int main(){
	s=s2=" ";
	for(int i=1;i<=n;++i){
		a=r();s+=(a+'0');s2+=(i+'0');
	}if(s==s2){cout<<0;return 0;}
	int x=1,y=1,nn=k/2;t1[x]=s;t2[x]=0;sb[s]=1;
	while(x<=y){
		s3=t1[x++];
		for(int i=1;i<=n-k+1;++i){
			s=s3;
			for(int j=1;j<=nn;++j)swap(s[i+j-1],s[i+k-j]);
			
			if(s==s2){
				cout<<t2[x-1]+1;return 0;
			}
			if(sb[s]!=1){
				t1[++y]=s;t2[y]=t2[x-1]+1;
				sb[s]=1;
			}
		}
	}
	cout<<-1;
}