Problem:
Score : 300 points

Problem Statement

Takahashi, Nakahashi and Hikuhashi have integers A, B and C, respectively.
After repeating the following operation K times, find the integer Takahashi will get minus the integer Nakahashi will get:

Each of them simultaneously calculate the sum of the integers that the other two people have, then replace his own integer with the result.

However, if the absolute value of the answer exceeds 10^{18}, print Unfair instead.

Constraints

1 \leq A,B,C \leq 10^9

0 \leq K \leq 10^{18}

All values in input are integers.

Input

Input is given from Standard Input in the following format:

A B C K

Output

Print the integer Takahashi will get minus the integer Nakahashi will get, after repeating the following operation K times.
If the absolute value of the answer exceeds 10^{18}, print Unfair instead.

Sample Input 1

1 2 3 1

Sample Output 1

1

After one operation, Takahashi, Nakahashi and Hikuhashi have 5, 4 and 3, respectively. We should print 5-4=1.

Sample Input 2

2 3 2 0

Sample Output 2

-1

Sample Input 3

1000000000 1000000000 1000000000 1000000000000000000

Sample Output 3

0

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

/*
URL https://
SCORE 0
AC false
WA false
TLE false
MLE false
TASK_TYPE
FAILURE_TYPE
NOTES
*/
#include <iostream>
#include <cstdint>
#include <utility>
#include <tuple>
#include <vector>
#include <stack>
#include <queue>
#include <unordered_map>
#include <unordered_set>
#include <map>
#include <set>
#include <algorithm>
#include <limits>
#include <numeric>
#include <iomanip>
#include <type_traits>
#include <functional>

using namespace std; // TODO Remove this

/* macros */
// loops
#define REP(i, n) for (i64 i = 0; i < static_cast<decltype(i)>(n); ++i)
#define REPR(i, n) for (i64 i = (n) - 1; i >= static_cast<decltype(i)>(0); --i)
#define FOR(i, n, m) for (i64 i = (n); i < static_cast<decltype(i)>(m); ++i)
#define FORR(i, n, m) for (i64 i = (m) - 1; i >= static_cast<decltype(i)>(n); --i)
#define EACH(x, xs) for (auto &x: (xs))
#define EACH_V(x, xs) for (auto x: (xs))

// helpers
#define CTR(x) (x).begin(), (x).end()

/* internal code */
namespace internal {
    /* utils for std::tuple */
    namespace tuple_utils {
        template<size_t...>
        struct seq {};

        template<size_t N, size_t... Is>
        struct gen_seq : gen_seq<N - 1, N - 1, Is...> {};

        template<size_t... Is>
        struct gen_seq<0, Is...> : seq<Is...> {};

        template<class Tuple, size_t... Is>
        void read(istream &stream, Tuple &t, seq<Is...>) {
            static_cast<void>((int[]) {0, (void(stream >> get<Is>(t)), 0)...});
        }

        template<class Tuple, size_t... Is>
        void print(ostream &stream, Tuple const &t, seq<Is...>) {
            static_cast<void>((int[]) {0, (void(stream << (Is == 0 ? "" : ", ") << get<Is>(t)), 0)...});
        }

        template<size_t I, class F, class A, class... Elems>
        struct ForEach {
            void operator()(A &arg, tuple<Elems...> const &t) const {
                F()(arg, get<I>(t));
                ForEach<I - 1, F, A, Elems...>()(arg, t);
            }

            void operator()(A &arg, tuple<Elems...> &t) const {
                F()(arg, get<I>(t));
                ForEach<I - 1, F, A, Elems...>()(arg, t);
            }
        };

        template<class F, class A, class... Elems>
        struct ForEach<0, F, A, Elems...> {
            void operator()(A &arg, tuple<Elems...> const &t) const {
                F()(arg, get<0>(t));
            }

            void operator()(A &arg, tuple<Elems...> &t) const {
                F()(arg, get<0>(t));
            }
        };

        template<class F, class A, class... Elems>
        void for_each(A &arg, tuple<Elems...> const &t) {
            ForEach<tuple_size<tuple<Elems...>>::value - 1, F, A, Elems...>()(arg, t);
        }

        template<class F, class A, class... Elems>
        void for_each(A &arg, tuple<Elems...> &t) {
            ForEach<tuple_size<tuple<Elems...>>::value - 1, F, A, Elems...>()(arg, t);
        }

        struct hash_for_element {
            template<class V>
            void operator()(size_t &size, const V &v) const {
                size ^= hash<V>()(v);
            }
        };
    }

    /* utils for std::vector */
    namespace vector_utils {
        template <typename V, int N>
        struct matrix_t {
            using type = std::vector<typename matrix_t<V, N-1>::type>;
        };
        template <typename V>
        struct matrix_t<V, 0> {
            using type = V;
        };

        template <typename V, int N>
        using Matrix = typename internal::vector_utils::matrix_t<V, N>::type;

        template <typename V, typename It, int N>
        struct matrix_helper {
            static Matrix<V, N> create(const It &begin, const It &end, const V &default_value) {
                return Matrix<V, N>(*begin, matrix_helper<V, It, N - 1>::create(begin + 1, end, default_value));
            }
        };
        template <typename V, typename It>
        struct matrix_helper<V, It, 0> {
            static Matrix<V, 0> create(const It &begin, const It &end, const V &default_value) {
                return default_value;
            }
        };
    }
}


/* Primitive types */
using i8  =  int8_t; using  u8 =  uint8_t;
using i16 = int16_t; using u16 = uint16_t;
using i32 = int32_t; using u32 = uint32_t;
using i64 = int64_t; using u64 = uint64_t;
using usize = size_t;

/* Data structure type */
template <class T> using V = vector<T>; // TODO Replace with Vector
template <typename T> using Vector = internal::vector_utils::Matrix<T, 1>;
template <typename T, int N> using Matrix = internal::vector_utils::Matrix<T, N>;
template <typename V> using OrderedSet = std::set<V>;
template <typename V> using HashSet = std::unordered_set<V>;
template <typename K, typename V> using OrderedMap = std::map<K, V>;
template <typename K, typename V> using HashMap = std::unordered_map<K, V>;

/* utils for std::vector */
template <typename V>
std::vector<V> make_pre_allocated_vector(size_t N) {
    std::vector<V> retval;
    retval.reserve(N);
    return retval;
}

template <class V, int N>
Matrix<V, N> make_matrix(const std::array<size_t, N> &shape, V default_value = V()) {
    return internal::vector_utils::matrix_helper<V, decltype(shape.begin()), N>::create(shape.begin(), shape.end(), default_value);
}
/* utils for STL containers */
template <class Iterator>
struct Container {
    Container(const Iterator &begin, const Iterator &end) : m_begin(begin), m_end(end) {}
    const Iterator& begin() const {
        return this->m_begin;
    }
    const Iterator& end() const {
        return this->m_end;
    }
    Iterator m_begin;
    Iterator m_end;
};

/* utils for STL iterators */
template <typename Iterator, typename F>
struct MappedIterator {
    MappedIterator(const Iterator &it, const F &function) : it(it), function(function) {}
    auto operator *() const {
        return this->function(this->it);
    }
    void operator++() { ++this->it; }
    void operator+=(size_t n) { this->it += n; }
    auto operator+(size_t n) const {
        return MappedIterator<Iterator, F>(this->it + n, this->function);
    }
    bool operator==(const MappedIterator<Iterator, F> &rhs) const {
        return this->it == rhs.it;
    }
    bool operator!=(const MappedIterator<Iterator, F> &rhs) const {
        return !(*this == rhs);
    }
private:
    Iterator it;
    F function;
};
template <typename Iterator, typename P>
struct FilteredIterator {
    FilteredIterator(const Iterator &it, const Iterator &end, const P &predicate)
            : it(it), end(end), predicate(predicate) {
        if (this->it != end) {
            if (!predicate(this->it)) {
                this->increment();
            }
        }
    }
    decltype(auto) operator *() const {
        return *this->it;
    }
    auto operator ->() const {
        return this->it;
    }
    void operator++() {
        this->increment();
    }
    void operator+=(size_t n) {
        REP (i, n) {
            this->increment();
        }
    }
    auto operator+(size_t n) const {
        auto retval = *this;
        retval += n;
        return retval;
    }
    bool operator==(const FilteredIterator<Iterator, P> &rhs) const {
        return this->it == rhs.it;
    }
    bool operator!=(const FilteredIterator<Iterator, P> &rhs) const {
        return !(*this == rhs);
    }
private:
    void increment() {
        if (this->it == this->end) {
            return ;
        }
        ++this->it;
        while (this->it != this->end && !this->predicate(this->it)) {
            ++this->it;
        }
    }
    Iterator it;
    Iterator end;
    P predicate;
};
template <typename Iterator, typename ElementIterator>
struct FlattenedIterator {
    FlattenedIterator(const Iterator &it, const Iterator &end) : it(make_pair(it, ElementIterator())), end(end) {
        if (this->it.first != this->end) {
            this->it.second = it->begin();
        }
    }
    decltype(auto) operator *() const {
        return this->it;
    }
    const pair<Iterator, ElementIterator> *operator ->() const {
        return &this->it;
    }
    void operator++() {
        this->increment();
    }
    void operator+=(size_t n) {
        REP (i, n) {
            this->increment();
        }
    }
    auto operator+(size_t n) const {
        auto retval = *this;
        retval += n;
        return retval;
    }
    bool operator==(const FlattenedIterator<Iterator, ElementIterator> &rhs) const {
        if (this->it.first != rhs.it.first) {
            return false;
        }
        if (this->it.first == this->end || rhs.it.first == rhs.end) {
            if (this->end == rhs.end) {
                return true;
            } else {
                return false;
            }
        } else {
            return this->it.second == rhs.it.second;
        }
    }
    bool operator!=(const FlattenedIterator<Iterator, ElementIterator> &rhs) const {
        return !(*this == rhs);
    }
private:
    void increment() {
        auto is_end = [this]() {
            return this->it.first == this->end;
        };
        auto is_valid = [this, &is_end]() {
            if (is_end()) return false;
            if (this->it.second == this->it.first->end()) return false;
            return true;
        };

        auto increment = [this]() {
            if (this->it.second == this->it.first->end()) {
                ++this->it.first;
                if (this->it.first != this->end) {
                    this->it.second = this->it.first->begin();
                }
            } else {
                ++this->it.second;
            }
        };

        increment();
        while (!is_end() && !is_valid()) {
            increment();
        }

    }
    pair<Iterator, ElementIterator> it;
    Iterator end;
};

template <typename C, typename F>
auto iterator_map(const C &c, F function) {
    using Iterator = std::remove_const_t<std::remove_reference_t<decltype(c.begin())>>;
    return Container<MappedIterator<Iterator, F>>(MappedIterator<Iterator, F>(c.begin(), function),
            MappedIterator<Iterator, F>(c.end(), function));
}

template <typename C, typename P>
auto iterator_filter(const C &c, P predicate) {
    using Iterator = std::remove_const_t<std::remove_reference_t<decltype(c.begin())>>;
    return Container<FilteredIterator<Iterator, P>>(FilteredIterator<Iterator, P>(c.begin(), c.end(), predicate),
                                                    FilteredIterator<Iterator, P>(c.end(), c.end(), predicate));
}

template <typename C>
auto iterator_flatten(const C &c) {
    using Iterator = std::remove_const_t<std::remove_reference_t<decltype(c.begin())>>;
    using ElementIterator = std::remove_const_t<std::remove_reference_t<decltype(c.begin()->begin())>>;
    return Container<FlattenedIterator<Iterator, ElementIterator>>(
            FlattenedIterator<Iterator, ElementIterator>(c.begin(), c.end()),
            FlattenedIterator<Iterator, ElementIterator>(c.end(), c.end()));
}

/* utils for STL iterators (TODO deprecated because it is too complex) */
template <class Functions>
struct BaseIterator {
    using State = typename Functions::State;

    BaseIterator(const State &state, const Functions &func) : state(state), func(func) {
        while (!this->is_end() && !this->is_valid()) {
            this->next();
        }
    }
    BaseIterator(const State &state) : state(state), func() {
        while (!this->is_end() && !this->is_valid()) {
            this->next();
        }
    }

    decltype(auto) operator*() {
        return this->func.get_value(this->state);
    }

    decltype(auto) operator*() const {
        return this->func.get_value(this->state);
    }

    BaseIterator &operator++() {
        if (this->is_end()) {
            return *this;
        }

        this->next();
        while (!this->is_end() && !this->is_valid()) {
            this->next();
        }

        return *this;
    }

    BaseIterator &operator--() {
        if (this->is_begin()) {
            return *this;
        }

        this->previous();
        while (!this->is_begin() && !this->is_valid()) {
            this->previous();
        }

        return *this;
    }

    bool operator==(const BaseIterator<Functions> &rhs) const {
        return this->state == rhs.state;
    }
    bool operator!=(const BaseIterator<Functions> &rhs) const {
        return !(*this == rhs);
    }

    bool is_begin() const {
        return this->func.is_begin(this->state);
    }
    bool is_end() const {
        return this->func.is_end(this->state);
    }
    const State &get_state() const {
        return this->state;
    }
private:
    bool is_valid() const {
        return this->func.is_valid(this->state);
    }
    void next() {
        this->func.next(this->state);
    }
    void previous() {
        this->func.previous(this->state);
    }
    State state;
    Functions func;
};

/* input */
template <class F, class S>
istream &operator>>(istream &stream, pair<F, S> &pair) {
    stream >> pair.first;
    stream >> pair.second;
    return stream;
}
template <class ...Args>
istream &operator>>(istream &stream, tuple<Args...> &tuple) {
    internal::tuple_utils::read(stream, tuple, internal::tuple_utils::gen_seq<sizeof...(Args)>());
    return stream;
}

template <class T>
T read() {
    T t;
    cin >> t;
    return t;
}
template <class F, class S>
pair<F, S> read() {
    pair<F, S> p;
    cin >> p;
    return p;
}
template <class T1, class T2, class T3, class ...Args>
tuple<T1, T2, T3, Args...> read() {
    tuple<T1, T2, T3, Args...> t;
    cin >> t;
    return t;
}
template <typename T, typename F = std::function<T()>>
Vector<T> read(const usize length, F r) {
    auto retval = make_pre_allocated_vector<T>(length);
    REP (i, length) {
        retval.emplace_back(r());
    }
    return retval;
}

template <class T>
Vector<T> read(const usize length) {
    return read<T>(length, [] { return read<T>(); });
}
template <class F, class S>
Vector<pair<F, S>> read(const usize length) {
    return read<pair<F, S>>(length);
}
template <class T1, class T2, class T3, class ...Args>
Vector<tuple<T1, T2, T3, Args...>> read(const usize length) {
    return read<tuple<T1, T2, T3, Args...>>(length);
}

/* debug output */
template <class F, class S>
ostream &operator<<(ostream& stream, const pair<F, S> &pair) {
    stream << "{" << pair.first << ", " << pair.second << "}";
    return stream;
}
template <class ...Args>
ostream &operator<<(ostream& stream, const tuple<Args...> &tuple) {
    stream << "{";
    internal::tuple_utils::print(stream, tuple, internal::tuple_utils::gen_seq<sizeof...(Args)>());
    stream << "}";
    return stream;
}

template <typename T>
void dump(const T& t) {
    std::cerr << t << std::endl;
}
template <typename F, typename S>
void dump(const pair<F, S>& p) {
    std::cerr << p << std::endl;
}
template <class ...Args>
void dump(const tuple<Args...> &tuple) {
    std::cerr << tuple << std::endl;
}
template <typename V1, typename V2, typename ...Args>
void dump(const V1 &v1, const V2 &v2, const Args&... args) {
    const auto x = std::make_tuple(v1, v2, args...);
    internal::tuple_utils::print(std::cerr, x, internal::tuple_utils::gen_seq<sizeof...(Args) + 2>());
    std::cerr << std::endl;
}

template <typename T>
void dump_matrix(const Matrix<T, 0> & matrix) {
    dump(matrix);
}
template <typename T>
void dump_matrix(const Matrix<T, 1> & matrix) {
    EACH (x, matrix) {
        std::cerr << x << " ";
    }
    std::cerr << std::endl;
}
template <typename T>
void dump_matrix(const Matrix<T, 2> & matrix) {
    EACH (x, matrix) {
        dump_matrix<T>(x);
    }
}

template <typename C>
void dump_set(const C &container) {
    EACH(c, container) {
        dump(c);
    }
}
template <typename C>
void dump_seq(const C &container) {
    int index = 0;
    EACH(c, container) {
        dump(index, c);
        index += 1;
    }
}
template <typename C>
void dump_map(const C &container) {
    EACH(c, container) {
        const auto &key = c.first;
        const auto &value = c.second;
        std::cerr << key << "\t:" << value << std::endl;
    }
}

// Hash
namespace std {
    template <class F, class S>
    struct hash<pair<F, S>> {
        size_t operator ()(const pair<F, S> &p) const {
            return hash<F>()(p.first) ^ hash<S>()(p.second);
        }
    };

    template <class ...Args>
    struct hash<tuple<Args...>> {
        size_t operator ()(const tuple<Args...> &t) const {
            size_t retval = 0;

            internal::tuple_utils::for_each<internal::tuple_utils::hash_for_element, size_t, Args...>(retval, t);

            return retval;
        }
    };
}
#define MAIN
void body();

// main function (DO NOT EDIT)
int main (int argc, char **argv) {
    cin.tie(0);
    ios_base::sync_with_stdio(false);

    cout << fixed;
    body();

    return 0;
}

void body() {
    auto A = read<i64>();
    auto B = read<i64>();
    auto C = read<i64>();
    auto K = read<i64>();

    if (K % 2 == 0) {
        auto ans = A - B;
        if (std::abs(ans) > 1e18) {
            cout << "Unfair" << endl;
        } else {
            cout << A - B << endl;
        }
    } else {
        auto ans = B - A;
        if (std::abs(ans) > 1e18) {
            cout << "Unfair" << endl;
        } else {
            cout << B - A << endl;
        }
    }
}