#include <algorithm>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <valarray>
#include <vector>

#define N 1000
// #define MOD 1000000007
#define MOD 1000000000000
#define pe(str) return cout<<(str)<<endl,0
#define px(str) {cout<<(str)<<endl;exit(0);}
#define re(i,n) for(int i=0;i<(n);i++)
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define fe(i,n,f) for_each(i,n,f)
#define bw(a,b,c) (((a)<=(b))&&((b)<=(c)))
using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int,int> pii;

template<class InputIterator>
void dump(InputIterator first,InputIterator last,char delim=' '){
	for(InputIterator it=first;it!=last;it++){
		if(it!=first)cout<<delim;
		cout<<*it;
	}
}

int main(void){
	int n,k,a[1001];
	int c=0;
	cin>>n>>k;
	re(i,n)cin>>a[i];
	re(i,n){
		re(j,n){
			if(j+k>=n)break;
			if(a[j]>a[j+k]){
				swap(a[j],a[j+k]);
				c++;
			}
		}
	}
	cout<<(is_sorted(a,a+n)?c:-1)<<endl;
	return 0;
}