foo.c: In function 'sum':
foo.c:3:5: warning: type of 'a' defaults to 'int' [-Wimplicit-int]
 int sum(a, b, c)
     ^~~
foo.c:3:5: warning: type of 'b' defaults to 'int' [-Wimplicit-int]
foo.c:3:5: warning: type of 'c' defaults to 'int' [-Wimplicit-int]
foo.c: In function 'main':
foo.c:17:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d%d%d", &a, &b, &c);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:23:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d", &choose);
   ^~~~~~~~~~~~~~~~~~~~

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