#include <math.h>

void main() {
    double a = 0;
    a += 5e308;
    a -= 3.5e308;

    if (a == INFINITY) {
        printf("1   %en", a);
    }
    else
        printf("0   %en", a);
}

Here, the first print line is what I expect but why is the second print statement executed?
I think a should be positive infinity as infinity -3.5e308 should be positive infinity, right in line 4?

I have already tried to execute it but the output is:

0   -1.#IND00e+000

I would like to know where my logic goes wrong.