import java.util.*;
import java.lang.*;
import java.io.*;

class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		int e, f, c;
		Scanner in = new Scanner(System.in);
		e = in.nextInt();
		f = in.nextInt();
		c = in.nextInt();
		System.out.print((int)((e + f)/(c - 1 + 0.0001)));
		
	}
}