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 a = read.to!long;
	long b = read.to!long;
	long c = read.to!long;
	
	long k = read.to!long;
	
	long ans;
	if(k % 2) ans = b - a;
	else ans = a - b;
	
	if(ans > 1_000_000_000_000_000_000) writeln("Unfair");
	else ans.writeln;
	
	}