foo.c: In function 'main':
foo.c:12:3: error: 'sum' undeclared (first use in this function)
   sum+=1;
   ^~~
foo.c:12:3: note: each undeclared identifier is reported only once for each function it appears in
foo.c:6:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d",&n,&k);
  ^~~~~~~~~~~~~~~~~~~
foo.c:10:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d%d",&x,&y);
   ^~~~~~~~~~~~~~~~~~~

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