foo.c: In function 'main':
foo.c:12:2: warning: statement with no effect [-Wunused-value]
  for(n;n<=k2;n++)
  ^~~
foo.c:9:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d",&n, &k2);
  ^~~~~~~~~~~~~~~~~~~~~
foo.c:32:6: warning: 'f3' may be used uninitialized in this function [-Wmaybe-uninitialized]
   sum+=f3;
   ~~~^~~~

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