int number = 0;
int main() {
fork()
if it is child {number--, return 0}
if it is parent {number++, wait till child return, print number}}

int number = 0;
t1() {number--}
t2() {number++}
main() {
    createThread(pass t1)
    createThread(pass t2)
    wait till both complete
    print number
}