C++ :: Why Two Scanf Needed To Read Character

Below given is the code, which allocates memory for a structure dynamically and stores value in its member. The problem is in the last scanf statement which reads 'ch'. The code will be in infinite loop as it doesnt executes the last scanf statement. The solution for this is (i use this) to add one more similar scanf statement for 'ch' in the very next line. If i do so, it executes the statement, reads 'ch' and then continues.

I want to know why it behaves like that..

struct student{
int usn;
};
int main(){
char ch;
struct student *s;
s=(struct student *)malloc(sizeof(struct student));

[Code] ....