readf interferes with readln

[Bastiaan Veelo via Digitalmars-d-learn]

Hi,

I am having trouble explaining the following to someone learning 
D. Can someone explain why readln has different behaviour when it 
is preceded by readf?

Suppose we want to not end the program before the user presses 
Enter by having readln at the end of main():

```
import std.stdio;

void main()
{
    int num;
    write("Give a number ");
    readf(" %s", num);
    writeln("Thanks");
    readln;
    readln;
}
```

In this example this requires twice readln. When you comment out 
readf, you need readln only once.

Thanks!