#include <iostream>
#include <vector>
#include <cstdio>
#include <sstream>
#include <map>
#include <string>
#include <algorithm>
#include <queue>
#include <cmath>
#include <functional>
#include <set>
#include <ctime>
#include <random>
#include <chrono>
#include <cassert>
#include <tuple>
#include <utility>
using namespace std;

namespace {
  using Integer = long long; //__int128;
  template<class T, class S> istream& operator >> (istream& is, pair<T,S>& p){return is >> p.first >> p.second;}
  template<class T> istream& operator >> (istream& is, vector<T>& vec){for(T& val: vec) is >> val; return is;}
  template<class T> istream& operator ,  (istream& is, T& val){ return is >> val;}
  template<class T, class S> ostream& operator << (ostream& os, const pair<T,S>& p){return os << p.first << " " << p.second;}
  template<class T> ostream& operator << (ostream& os, const vector<T>& vec){for(size_t i=0; i<vec.size(); i++) os << vec[i] << (i==vec.size()-1?"":" "); return os;}
  template<class T> ostream& operator ,  (ostream& os, const T& val){ return os << " " << val;}

  template<class H> void print(const H& head){ cout << head; }
  template<class H, class ... T> void print(const H& head, const T& ... tail){ cout << head << " "; print(tail...); }
  template<class ... T> void println(const T& ... values){ print(values...); cout << endl; }

  template<class H> void eprint(const H& head){ cerr << head; }
  template<class H, class ... T> void eprint(const H& head, const T& ... tail){ cerr << head << " "; eprint(tail...); }
  template<class ... T> void eprintln(const T& ... values){ eprint(values...); cerr << endl; }

  class range{ Integer start_, end_, step_; public: struct range_iterator{ Integer val, step_; range_iterator(Integer v, Integer step) : val(v), step_(step) {} Integer operator * (){return val;} void operator ++ (){val += step_;} bool operator != (range_iterator& x){return step_ > 0 ? val < x.val : val > x.val;} }; range(Integer len) : start_(0), end_(len), step_(1) {} range(Integer start, Integer end) : start_(start), end_(end), step_(1) {} range(Integer start, Integer end, Integer step) : start_(start), end_(end), step_(step) {} range_iterator begin(){ return range_iterator(start_, step_); } range_iterator   end(){ return range_iterator(  end_, step_); } };

  inline string operator "" _s (const char* str, size_t size){ return move(string(str)); }
  constexpr Integer my_pow(Integer x, Integer k, Integer z=1){return k==0 ? z : k==1 ? z*x : (k&1) ? my_pow(x*x,k>>1,z*x) : my_pow(x*x,k>>1,z);}
  constexpr Integer my_pow_mod(Integer x, Integer k, Integer M, Integer z=1){return k==0 ? z%M : k==1 ? z*x%M : (k&1) ? my_pow_mod(x*x%M,k>>1,M,z*x%M) : my_pow_mod(x*x%M,k>>1,M,z);}
  constexpr unsigned long long operator "" _ten (unsigned long long value){ return my_pow(10,value); }

  inline int k_bit(Integer x, int k){return (x>>k)&1;} //0-indexed

  mt19937 mt(chrono::duration_cast<chrono::nanoseconds>(chrono::steady_clock::now().time_since_epoch()).count());

  template<class T> string join(const vector<T>& v, const string& sep){ stringstream ss; for(size_t i=0; i<v.size(); i++){ if(i>0) ss << sep; ss << v[i]; } return ss.str(); }

  inline string operator * (string s, int k){ string ret; while(k){ if(k&1) ret += s; s += s; k >>= 1; } return ret; }
}
constexpr long long mod = 9_ten + 7;


//[n*p] * [p*m] => [n*m]
template<class T>
vector< vector<T> > multmat(const vector<vector<T> > &A, const vector<vector<T>> &B, int n, int p, int m){
  vector<vector<T> > C(n, vector<T>(m,0));
  for(int i=0; i<n; i++){
    for(int k=0; k<p; k++){
      for(int j=0; j<m; j++){
        C[i][j] += A[i][k] * B[k][j];
        //C[i][j] %= mod;
      }
    }
  }
  return C;
}

//A[n*n]^k 
template<class T>
vector< vector<T> > mat_pow(vector<vector<T> > A, long long k){
  int n = A.size();
  vector<vector<T> > ret(n, vector<T>(n, 0) );
  for(int i=0; i<n; i++){
    ret[i][i] = 1;
  }
  while(k>0){
    if(k&1) ret = multmat(A,ret, n,n,n);
    A = multmat(A,A, n,n,n);
    k>>=1;
  }
  return ret;
}

int main(){
  long long a,b,c,k;
  cin >> a,b,c,k;

  vector<vector<__int128>> m = {
    {0,1,1},
    {1,0,1},
    {1,1,0},
  };

  vector<vector<__int128>> x = {
    {a},
    {b},
    {c}
  };

  auto w = mat_pow(m, k);
  auto z = multmat(w, x, 3, 3, 1);

  __int128 ans = z[0][0] - z[1][0];

  if( ans <= (__int128)18_ten && -ans <= (__int128)18_ten ){
    long long ans_ = ans;
    println(ans_);
  }else{
    println("Unfair");
  }
  
  return 0;
}


Measured memory_bytes: 256