#include <bits/stdc++.h>
#define rep(i,a,n) for(int i=a;i<n;i++)
#define repb(i,a,b) for(int i=a;i>=b;i--)
#define all(a) a.begin(),a.end()
#define o(a) cout<<a<<endl
#define int long long
#define fi first
#define se second
using namespace std;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pii;

signed main(){
	int n,k;
	cin>>n>>k;
	vi a(n);
	rep(i,0,n) cin>>a[i];
	int ans=0,cnt=0;
	for(int i=0;;i++){
		if(n<=k) break;
		if(i>=n-k) i=0;
		if(a[i]>a[i+k]){
			ans++;
			swap(a[i],a[i+k]);
			cnt=0;
		}else cnt++;
		if(cnt>2*n) break;
	}
	bool f=true;
	rep(i,1,n) if(a[i-1]>a[i]) f=false;
	if(!f) cout<<-1<<endl;
	else cout<<ans<<endl;
}