#include <stdio.h>
int main()
 { FILE *f;  long offset=0L;    int ch;
   f=fopen("test.dat","r");
   while ((!fseek(f,offset++,0)) && ((ch=getc(f))!=EOF))
    { putc(ch,stdout);
      if (!fseek(f,-(offset+2),2)) putc(getc(f),stdout);
    }
   fclose(f);
   return 0;
 }
