#include <stdio.h>

#define MAXROW 500
#define MAXCOL 500

void readImage ( char fname[] , int *ht , int *wd , int data[][MAXCOL] )
{
   FILE *fp;
   char tmp[100];
   int lev, i, j;

   fp = (FILE *)fopen(fname,"r");
   if (fp == NULL) {
      fprintf(stderr,"Error opening input file...\n");
      exit(1);
   }
   fscanf(fp,"%s",tmp);
   fscanf(fp,"%d%d%d",wd,ht,&lev);
   for (i=0; i<*ht; ++i) for (j=0; j<*wd; ++j) fscanf(fp,"%d",&data[i][j]);
   fclose(fp);
   printf("%d x %d image read from \"%s\".\n", *wd, *ht, fname);
}

void procImage ( int ht , int wd , int src[][MAXCOL] , int dest[][MAXCOL] )
{
   int i, j;

   for (i=0; i<ht/2; ++i) for (j=0; j<wd/2; ++j) {
      dest[i][j] = ( src[2*i][2*j] + src[2*i][2*j+1] +
                     src[2*i+1][2*j] + src[2*i+1][2*j+1] ) / 4;
   }
}

void writeImage ( char fname[] , int ht , int wd , int data[][MAXCOL] )
{
   FILE *fp;
   int i, j;

   fp = (FILE *)fopen(fname,"w");
   if (fp == NULL) {
      fprintf(stderr,"Error opening output file...\n");
      exit(1);
   }
   fprintf(fp,"P2\n%d %d\n255\n",wd,ht);
   for (i=0; i<ht; ++i) for (j=0; j<wd; ++j) fprintf(fp,"%d\n",data[i][j]);
   fclose(fp);
   printf("%d x %d image saved as \"%s\".\n", wd, ht, fname);
}

int main ()
{
   char infname[100], outfname[100];
   int src[MAXROW][MAXCOL], dest[MAXROW][MAXCOL], sht, swd, dht, dwd;

   printf("Enter the name of the input file  : "); scanf("%s",infname);
   printf("Enter the name of the output file : "); scanf("%s",outfname);
   readImage(infname, &sht, &swd, src);
   dht = sht / 2; dwd = swd / 2;
   procImage(sht, swd, src, dest);
   writeImage(outfname, dht, dwd, dest);
}