Transform a RGB image into gray

Image Transformation

#include <fstream>
#include <iostream>
using namespace std;
//Transform a RGB image into gray.
int main()
{
    //ifstream cin("image_transformation.txt");
    int x,y;
    int index=1;
    while(cin>>x>>y)
    {
        if(x==0)
            break;
        int z=x*y;
        int c[z][3];
        for(int i=0;i<3;i++)
        {
            int num;
            for(int j=0;j<x*y;j++)
            {
                cin>>num;
                c[j][i]=num;
            }   
        }
        cout<<"Case "<<index<<":"<<endl;
        index++;
        for(int i=0;i<x;i++)
        {
            for(int j=0;j<y;j++)
            {
                int sum=0;
                for(int k=0;k<3;k++)
                    sum+=c[i*y+j][k];
                printf("%d",sum/3);
                if(j!=y-1) cout<<",";
            }   
            cout<<endl;
        }   
    }   
    return 0;
}