foo.cc: In function ‘int main()’:
foo.cc:11:20: error: expected ‘;’ before ‘for’
   11 |                 b=0
      |                    ^
      |                    ;
   12 |                 for     (i=2;i<=a-1;i++)
      |                 ~~~ 
foo.cc:12:31: warning: statement has no effect [-Wunused-value]
   12 |                 for     (i=2;i<=a-1;i++)
      |                              ~^~~~~
foo.cc:12:40: error: expected ‘;’ before ‘)’ token
   12 |                 for     (i=2;i<=a-1;i++)
      |                                        ^
      |                                        ;
foo.cc:15:25: error: ‘else’ without a previous ‘if’
   15 |                         else
      |                         ^~~~