Why I got "5" not "-5"

#include <stdio.h>

#define M(X,Y) X*Y
void main(){
	int a=2,b=3;

I put this code in photon and got “5”, I just wonder why I didn’t get “-5”.

How are you building your code?
Your code doesn’t build for me in Web IDE since it should be int main() (although you don’t implement a main() function as this is part of the system itself) and there is no printf() for the Photon.

If you post code that’s supposed to run on a (stock) Photon, then you should post the exact code you are actually testing.

Can you test again via Web IDE an this code

#define M(X,Y) X*Y

void setup() {
void loop() {
  int a=2, b=3;
  Serial.printlnf("%d", M(a+b,a-b));

and check the output via a serial terminal program?

BTW, if you unpack that macro you’d end up with this code

  Serial.printlnf("%d", a+b*a-b);

does this expalin the result?
You may want to rewrite your macro like this

#define M(X,Y) (X)*(Y)

Since M is a macro and not a function, it behaves as a+b*a-b, or 2+3*2-3, but since * has precedence over +, it’s grouped as 2+(3*2)-3 = 2 + 6 - 3 = 5.