int
foo(a,b)
	SV * a
	SV * b
    PREINIT:
      int aa, bb;
    CODE:
      aa = SvIOK(a) ? SvIV(a) : -1;
      bb = SvIOK(b) ? SvIV(b) : -1;
      RETVAL = aa * bb;
    OUTPUT:
      RETVAL