/in/foo.c:4:6: warning: return type of 'main' is not 'int' [-Wmain]
 void main()
      ^~~~
/in/foo.c: In function 'main':
/in/foo.c:9:11: warning: right-hand operand of comma expression has no effect [-Wunused-value]
    if(a=-N,a<N,a++)
           ^
/in/foo.c:10:11: warning: right-hand operand of comma expression has no effect [-Wunused-value]
     if(b=N,b>-N,b--)
           ^