## 実装

#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++ (i))
using ll = long long;
using namespace std;

template <typename T, size_t H, size_t W>
using matrix = array<array<T, W>, H>;
template <typename T, size_t A, size_t B, size_t C>
matrix<T, A, C> operator * (matrix<T, A, B> const & a, matrix<T, B, C> const & b) {
matrix<T, A, C> c = {};
REP (y, A) REP (z, B) REP (x, C) c[y][x] += a[y][z] * b[z][x];
return c;
}
template <typename T, size_t H, size_t W>
array<T, H> operator * (matrix<T, H, W> const & a, array<T, W> const & b) {
array<T, H> c = {};
REP (y, H) REP (z, W) c[y] += a[y][z] * b[z];
return c;
}
template <typename T, size_t N>
matrix<T, N, N> unit_matrix() {
matrix<T, N, N> a = {};
REP (i, N) a[i][i] = 1;
return a;
}
template <typename T, size_t N>
matrix<T, N, N> powmat(matrix<T, N, N> x, ll k) {
matrix<T, N, N> y = unit_matrix<T, N>();
for (; k; k >>= 1) {
if (k & 1) y = y * x;
x = x * x;
}
return y;
}

template <typename T>
T solve1(int a, int b, int c, ll k) {
matrix<T, 3, 3> f = {{
{{ 0, 1, 1 }},
{{ 1, 0, 1 }},
{{ 1, 1, 0 }},
}};
array<T, 3> x = {{ (T)a, (T)b, (T)c }};
array<T, 3> y = powmat(f, k) * x;
return y[0] - y[1];
}
ll solve(int a, int b, int c, ll k) {
auto x = solve1<long double>(a, b, c, k);
if (abs(x) > 1e18 * 1.1) return LLONG_MAX;
auto y = solve1<ll>(a, b, c, k);
if (abs(y) > 1e18) return LLONG_MAX;
return y;
}

int main() {
int a, b, c; ll k; cin >> a >> b >> c >> k;
ll ans = solve(a, b, c, k);
if (ans == LLONG_MAX) {
cout << "Unfair" << endl;
} else {
cout << ans << endl;
}
return 0;
}