Codeforces Round #177 (Div. 2), problem: (B) Polo the Penguin and Matrix

#pragma GCC optimize("Ofast")
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")

#include<bits/stdc++.h>

using namespace std;
#define pb push_back
#define pii pair<int,int>
#define pli pair<long long int,int>
#define pbi pair<bool,int>
#define pob pop_back
#define all(a) a.begin(),a.end()
#define fio ios_base::sync_with_stdio(false); cin.tie(0);cout.tie(0);
#define i64 long long int
#define mem(x,y) memset(x,y,sizeof(x))
#define fill(arr,b) fill(arr, arr+sizeof(arr)/sizeof(arr[0]), b)
#define maxarr(n,arr,m) m =-1e9;for(int i=0;i<n;i++)if(arr[i]>m) m=arr[i];
#define read freopen("input.txt","r",stdin);

const double pi = acos(-1.0);
const int maxn = (i64)1e9+7;

int arr[10005];
int main()
{
    int n,m,d;
    scanf("%d%d%d",&n,&m,&d);
    int x=n*m;
    for(int i=0;i<x;i++) scanf("%d",&arr[i]);
    int k=arr[0]%d;
    arr[0]=arr[0]/d;
    bool fg=true;
    for(int i=1;i<x;i++)
    {
        if(arr[i]%d!=k) {fg=false;break;}
        else arr[i] /= d;
    }
    if(!fg){printf("-1");return 0;}
    sort(arr,arr+x);
    k=x/2;
    n=0;
    for(int i=0;i<x;i++)
    {
        n+=(abs(arr[i]-arr[k]));
    }
    printf("%d",n);
    return 0;
}