foo.cc: In function 'int main()':
foo.cc:16:2: error: unable to find numeric literal operator 'operator""cm'
   16 |  1cm=a1/b*b1<<endl;
      |  ^~~
foo.cc:16:2: note: use '-fext-numeric-literals' to enable more built-in suffixes
foo.cc:16:13: error: invalid operands of types 'int' and '<unresolved overloaded function type>' to binary 'operator<<'
   16 |  1cm=a1/b*b1<<endl;
      |      ~~~~~~~^~~~~~

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