#include<iostream>
#include<iomanip>
#include<vector>
#include<map>
#include<set>
#include<algorithm>
#include<numeric>
#include<limits>
#include<bitset>
#include<functional>
#include<type_traits>
#include<queue>
#include<stack>
#include<array>
#include<random>
#include<utility>
#include<cstdlib>
#include<cstring>
#include<string>
#include<ctime>
#include <type_traits>


using namespace std;

using ll = long long int;
using ull = long long unsigned;
using ld = long double;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
const double PI = acos(-1.0);
#define YES(n) cout << ((n) ? "YES" : "NO") << endl;
#define Yes(n) cout << ((n) ? "Yes" : "No") << endl;
#define Else(a,b,c) a!=b && b!=c && c!=a
#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 Sort(v) sort(all(v));
#define Rsort(v,type) sort(all(v),greater<type>());

const long MOD = pow(10, 9) + 7;
const double pai = 3.14159265358979;

char alfa[] = { 'A','B' ,'C','D' ,'E' ,'F' ,'G' ,'H' ,'I' ,'J' ,'K' ,'L' ,'M' ,'N' ,'O' ,'P' ,'Q' ,'R' ,'S' ,'T' ,'U' ,'V' ,'W' ,'X' ,'Y','Z' };

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

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;
			}
		}
	}
}

unsigned GetDigit(double num) {
	return log10(num) + 1;
}

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;
}

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;
	}
};



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

void Main();
inline void DebugMain() {
	while (true) {
		Main();
	}
}

int main() {
	cin.tie(nullptr);
	ios_base::sync_with_stdio(false);
	cout << fixed << setprecision(15);

	bool debug = false;
	if (debug) {
		cout << "\x1b[41m" << "Debug Mode " << endl;
	}
	(debug) ? DebugMain() : Main();
	return 0;
}

int parmitation(ll N) {
	ll ans = 1;
	for (ll i = 1; i <= N; i++) {
		ans *= i;
	}

	return ans;
}

int SemiCommonMultiple(ll A, ll ANum, ll B, ll BNum, ll diff, ll max) {
	ll calc;
	calc = diff + B * BNum - A * ANum;

	if (diff + B * BNum > max) {
		return 0;
	}
	else {
		if (calc < 0) {
			SemiCommonMultiple(A, ANum, B, ++BNum, diff, max);
		}
		else if (calc > 0) {
			SemiCommonMultiple(A, ++ANum, B, BNum, diff, max);
		}
		else {
			return A * (ANum + 0.5);
		}
	}
}

ll  CaracalVsMonstor(ll H, ll sum, ll count) {
	if (H == 0) {
		return sum;
	}


	H = H / 2;
	sum = sum + count;

	count = count * 2;

	return CaracalVsMonstor(H, sum, count);

}

class ibis {
public:
	ll A;
	ll B;
	ld spec;

	bool operator<(const ibis& another) const
	{
		return spec < another.spec;
	};
};


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

	return gcd(b, a);
}



inline void Main() {
	ll X, Y, N, M, K, H;
	vector<ll> hoge;
	ll ans, temp = 0;
	ll sum = 0, count = 0;
	double avg = 0.0;
	string S;
	string T;
	string Ans = "";

	ll A, B, C;

	cin >> A >> B >> C >> K;

	if (A  > K) {
		cout << K;
	}
	else if (A + B > K) {
		cout << A;
	}
	else {
		cout << A - (K - (A + B));
	}
}

Measured memory_bytes: 3680