foo.c:3:1: warning: return type defaults to 'int' [-Wimplicit-int]
 main (void) {
 ^~~~
foo.c: In function 'main':
foo.c:4:12: warning: unused variable 'b' [-Wunused-variable]
  int i, a, b;
            ^
foo.c:5:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d", &a);
  ^~~~~~~~~~~~~~~

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