#include"bits/stdc++.h"
#include <string>
using namespace std;

using ll = long long int;
using ull = unsigned long long int;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define int ll

const double PI = acos(-1.0);
const int MOD = (int)pow(10, 9) + 7;

#define YES(n) cout << ((n) ? "YES" : "NO") << endl;
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl;
#define all(v) v.begin(), v.end()
#define Ini(a) ll a; cin >> a;
#define Ind(a) double a; cin >> a;
#define Ins(a) string a; cin >> a;
#define Inv(a,type,n) vector<type> a;a.reserve(n);for(ll i=0;i<n;++i){type tmp;cin>>tmp;a.emplace_back(tmp);}
#define Out(Bool, T, F) cout << ((Bool) ? T : F) << endl;
#define Sort(v) sort(all(v));
#define Rsort(v,type) sort(all(v),greater<type>());

ll modinv(ll a, ll m) {
	ll b = m, u = 1, v = 0;
	while (b) {
		ll t = a / b;
		a -= t * b; swap(a, b);
		u -= t * v; swap(u, v);
	}
	u %= m;
	if (u < 0) u += m;
	return u;
}

inline void finish() {
	cout << -1 << endl;
	exit(0);
}

template <typename T>
inline void debug(vector<T> v) {
	if (v.size() == 0) {
		cout << "[]" << endl;
		return;
	}
	cout << "[";
	for (size_t i = 0; i < v.size() - 1; ++i) {
		cout << v[i] << ", ";
	}
	cout << v[v.size() - 1] << "]" << endl;
}

template <typename T, typename U>
inline void debug(map<T, U> m) {
	if (m.size() == 0) {
		cout << "[]" << endl;
		return;
	}
	for (pair<T, U> p : m) {
		cout << "[" << p.first << ", " << p.second << "], ";
	}
}

class Pos {
public:
	double x, y;
	Pos(double x, double y) {
		this->x = x;
		this->y = y;
	}
	Pos() {
		x = 0.0;
		y = 0.0;
	}

	bool isSamePos(Pos pos1, Pos pos2) {
		if (pos1.x == pos2.x && pos1.y == pos2.y) {
			return true;
		}
		return false;
	}
};

class Mod {
public:
	static inline ll Add(ll a, ll b) {
		return (a + b) % MOD;
	}

	static inline ll Sub(ll a, ll b) {
		ll ans = (a - b) % MOD;
		return (ans < 0) ? (ans + MOD) : ans;
	}

	static inline ll Mul(ll a, ll b) {
		return (a * b) % MOD;
	}
	
	static inline ll Div(ll a, ll b) {
		return (a % MOD) * modinv(b, MOD) % MOD;
	}
};

inline double distance(Pos p1, Pos p2) {
	return sqrt(pow(p1.x - p2.x, 2) + pow(p1.y - p2.y, 2));
}

inline ull Kaijo(int n) {
	ull ans = 1;
	for (int i = 2; i <= n; ++i) {
		ans *= i;
	}
	return ans;
}

template <typename T>
inline int GetKeta(T num) {
	return to_string(num).length();
}

void Main();
inline void DebugMain() {
	while (true) {
		Main();
		cout << endl;
	}
}

signed main() {
	bool debug = false;
	cin.tie(nullptr);
	ios_base::sync_with_stdio(false);
	cout << fixed << setprecision(15);

	if (debug) {
		cout << "\x1b[41m" << "Debug Mode このまま提出すると死" << endl;
	}
	try {
		(debug) ? DebugMain() : Main();
	}
	catch (exception& e) {
		cout << e.what() << endl;
	}

	return 0;
}

template <typename T>
inline bool isExistArray(vector<T>& v, T& str) {
	if (v.size() == 0) {
		return false;
	}
	for (string& s : v) {
		if (s == str) {
			return true;
		}
	}
	return false;
}

inline int ctoi(char c) {
	return (int)c - 48;
}

inline int find(vector<int>& v, int n) {
	for (size_t i = 0; i < v.size(); ++i) {
		if (v[i] == n) {
			return i;
		}
	}
	return -1;
}

inline bool isSameArray(vector<int>& v1, vector<int>& v2) {
	for (size_t i = 0; i < v1.size(); ++i) {
		if (v1[i] != v2[i]) {
			return false;
		}
	}
	return true;
}

inline bool isSeisu(double n) {

	if (std::floor(n) == n) {
		return true;
	}
	return false;
}

int gcd(int a, int b)
{
	if (a % b == 0)
	{
		return(b);
	}
	else
	{
		return(gcd(b, a % b));
	}
}

int lcm(int a, int b)
{
	return a * b / gcd(a, b);
}

inline ll Kiriage(ll a, ll b) {
	return (a + (b - 1)) / b;
}

template<class T>
inline void container_sort(std::vector<T>& v) {
	std::sort(all(v));
}
template<class T>
inline void container_sort(std::list<T>& v) {
	v.sort();
}

template<class T>
inline void Unique(T& v) {

	container_sort(v);
	v.erase(
		std::unique(
			v.begin(),
			v.end()),
		v.end()
	);
}

inline vector<bool> GetPrimeTable(int n) {
	vector<bool> primeTable(n + 1, true);
	int sqr = sqrt(n);
	primeTable[0] = false; //0は素数じゃない
	primeTable[1] = false; //1は素数じゃない
	for (int i = 2; i <= sqr; ++i) {
		if (primeTable[i]) {
			for (size_t j = i + i; j < primeTable.size(); j += i) {
				primeTable[j] = false;
			}
		}
	}
	return primeTable;
}

inline vector<ll> GetPrimeNumbers(int n) {
	vector<bool> primeTable = GetPrimeTable(n);
	vector<ll> result;
	if (n < 2) {
		result;
	}
	result.reserve(n / 2);
	for (size_t i = 0; i < primeTable.size(); ++i) {
		if (primeTable[i]) {
			result.push_back(i);
		}
	}
	result.shrink_to_fit();
	return result;
}

inline map<ll, ll> PrimeFact(ull n) {
	ull num = 2;
	map<ll, ll> result;
	while(true) {
		if (n % num == 0) {
			if (result.count(num) == 0) {
				result.insert(pair<ll, ll>{num, 1});
			}
			else {
				result[num]++;
			}
			n /= num;
		}
		else {
			if (++num > sqrt(n)) {
				if (result.size() == 0 || n != 1) {
					result.insert(pair<ll, ll>{n, 1});
				}
				return result;
			}
		}
	}
}

inline void Main() {
	Ini(A); Ini(B); Ini(C); Ini(K);
	int i;
	int sum = 0;
	if (A >= K) {
		cout << K;
		return;
	}
	for (i = 0; i < A; ++i) {
		if (i > K) {
			cout << sum;
			return;
		}
		sum++;
	}
	i += B;
	if (i > K) {
		cout << sum;
		return;
	}

	if (K - i > 0) {
		sum -= (K - i);
	}
	cout << sum;
}

Measured memory_bytes: 3644