[Beginner]Variable length arrays

[helxi via Digitalmars-d-learn]

I am trying to create an array which has a user defined size. 
However the following program is not compiling:

import std.stdio;

void main(){
     write("Enter your array size: ");
     int n;
     readf(" %s", &n);
     int[n] arr; //<-Error: variable input cannot be read at 
compile time
     writeln(arr);
}

1. What's causing this?
2. How can I get around this? I know I can always create a loop 
that appends value 'n' times.