import std.stdio, std.conv, std.string, std.bigint;
import std.math, std.random, std.datetime;
import std.array, std.range, std.algorithm, std.container;
string read(){ static string[] ss; while(!ss.length) ss = readln.chomp.split; string res = ss[0]; ss.popFront; return res; }

void main(){
	long n = read.to!long;
	long k = read.to!long;
	
	long[] as = readln.chomp.split.to!(long[]);
	
	long ans = ((n - 1) + (k - 1 - 1)) / (k - 1);
	
	ans.writeln;
	}