[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.