为什么死循环

#include<iostream> 
using namespace std;
int num,sum;
int main(){
cin>>num; 
while(num>1) {
if(num%2==0){
num/=2;
sum+=1;	
}	
if(num%2==1){
num=num*3+1;
sum+=1;	
}
}
cout<<sum;
return 0;
}