class Program{
static void Main(){
int? ni1 = null;
int? ni2 = 9;
int? ni3 = ni1 + ni2; // OK, ni3 is null.
int? ni4 = ni1 + 3; // OK, ni4 is null.
int? ni5 = ni2 + 3; // OK, ni5 is equal to 12.
ni1++; // OK, ni1 stays null.
ni2++; // OK, ni2 is now equal to 1.
}
}