#include <iostream>
#include <cmath>
#include <algorithm>
#include <vector>
#include <ctime>
#include <map>
using namespace std;

#define fi first
#define se second
#define TIME (1.0 * clock() / CLOCKS_PER_SEC)

const long long mod = 1e9 + 7;
const long long mod1 = 1e9 + 4321, mod2 = 1e9 + 321;
const long long MAXN = 2e5 + 5;
const long long inf = 1e18;
//---------------------------------------------------
long long n, k, need[MAXN], real[MAXN], ans;
map<long long, long long> pos;
int main() {
	cin >> n;
	for(int i =1 ; i <= n; i ++)
	{
		cin >> need[i];
		real[i] = need[i];
	}
	cin >> k;
	sort(need + 1, need + n + 1);
	for(int i =1 ; i <= n; i++)
	{
		pos[need[i]] = i;
	}
	for(int i = 1; i <= n; i ++)
	{
		if((pos[real[i]] - i) % k != 0)
		{
			cout << -1;
			return 0;
		}
		//cout << pos[real[i]] << " " << i << "\n";
		ans += abs(pos[real[i]] - i)/k;
	}
	cout << ans/2;
    cerr << "Time elapsed: " << TIME << " s.\n";
    return 0;
}