#include<bits/stdc++.h>
using namespace std;
const int MAXN = 1e4 + 5;
int a[MAXN];
int main()
{
	//freopen(".in", "r", stdin);
	//freopen(".out", "w", stdout);
	ios::sync_with_stdio(0);
	cin.tie(0), cout.tie(0);
	int n, m, d, gcd;
	cin >> n >> m >> d;
	cin >> a[1];
	gcd = a[1];
	for(int i = 2; i <= n * m; i ++)
	{
		cin >> a[i];
		gcd = __gcd(a[i], gcd);
	}
	if(gcd == 1 || d % gcd != 0)
	{
		cout << "-1";
		return 0;
	}
	sort(a + 1, a + n + 1);
	int tianxuanzhizi = a[n * m  / 2], ans = 0;
	for(int i = 1; i <= n * m; i ++)
		if(a[i] < tianxuanzhizi)
			ans += (tianxuanzhizi - a[i]) / d;
		else
			ans += (a[i] - tianxuanzhizi) / d;
	cout << ans;
	return 0;
}