foo.cc:2:6: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
 main()
      ^
foo.cc: In function 'int main()':
foo.cc:5:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d",&a);
 ~~~~~^~~~~~~~~