foo.c: In function 'main':
foo.c:6:2: error: expected ',' or ';' before 'scanf'
  scanf("%d", &a);
  ^~~~~
foo.c:17:2: error: expected ';' before '}' token
  }
  ^
foo.c:21:2: error: expected ';' before '}' token
  }
  ^
foo.c: At top level:
foo.c:24:12: error: expected declaration specifiers or '...' before string constant
     printf("hello, world\n");
            ^~~~~~~~~~~~~~~~
foo.c:25:1: error: expected identifier or '(' before '}' token
 }
 ^

[Hydro](https://hydro.ac)提供评测服务