#include <vector>
#include <list>
#include <map>
#include <set>
#include <deque>
#include <stack>
#include <queue>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>

using namespace std;

typedef long long ll;
typedef unsigned long long ull;

static const double EPS = 1e-8;
static const double PI = 4.0 * atan(1.0);
static const double PI2 = 8.0 * atan(1.0);

#define REP(i,n)	for(int i=0;i<(int)n;++i)
#define ALL(c)		(c).begin(),(c).end()
#define CLEAR(v)	memset(v,0,sizeof(v))
#define MP(a,b)		make_pair((a),(b))
#define ABS(a)		((a)>0?(a):-(a))
#define FOR(i,s,n)	for(int i=s;i<(int)n;++i)

int solve(vector<int> &arr) {
	int n = arr.size();
	int res = 0;
	--n;
	REP(i, n) REP(j, n - i) {
		if (arr[j] > arr[j + 1]) {
			++res;
			swap(arr[j], arr[j + 1]);
		}
	}
	return res;
}

int main(int argc, char **argv) {
	int N, K;
	cin >> N >> K;
	vector<vector<int>> arrs(K);
	REP(i, N) {
		int a;
		cin >> a;
		arrs[i % K].push_back(a);
	}
	int res = 0;
	REP(i, K) res += solve(arrs[i]);
	int prev = -1;
	REP(i, N) {
		if (arrs[i % K][i / K] < prev) {
			res = -1;
			break;
		}
		prev = arrs[i % K][i / K];
	}
	cout << res << endl;
	return 0;
}