#include <bits/stdc++.h>
#define int int64_t
#define REP(i, n) for (int i = 0; (i) < (int)(n); ++ (i))
#define REP3(i, m, n) for (int i = (m); (i) < (int)(n); ++ (i))
#define REP_R(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define REP3R(i, m, n) for (int i = (int)(n) - 1; (i) >= (int)(m); -- (i))
#define REPP(i, n) for (int i = 1; (i) < (int)(n+1); ++ (i))
#define REPR(i, n) for (int i = (int)(n) - 1; (i) >= 0; -- (i))
#define REPRP(i, n) for (int i = (int)(n); (i) >= 0; -- (i))
#define FOR(i, m, n) for(int i = m; i < n; i++)
#define FORP(i, m, n) for(int i = m; i < n+1; i++)
#define FORR(i, a, b) for(int i=(b)-1; i>=(a); i--)
#define ALL(v) v.begin(), v.end()
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(),(c).end())
#define SORTR(c) sort((c).begin(),(c).end(),greater<int>());
#define BIT(n) (1LL<<(n))
#define PCNT(x) __builtin_popcountll(x)
#define GCD(x,y) __gcd(x,y)
#define LCM(x,y) x/__gcd(x,y)*y
#define PB push_back
#define MP make_pair
#define endl "\n"
#define pa cout << ans << endl
#define p(x) cout << x << endl
#define pall(x) for(auto iter : x) p(iter)
#define yes cout << "Yes" << endl
#define no cout << "No" << endl
#define yn(x) cout << (x ? "Yes" : "No") << endl
using namespace std;
typedef long double LD;
typedef vector<int> VI;
typedef vector<vector<int> > VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef vector<PII> VPII;
using Graph = vector<vector<int> >;
using WeightedGraph = vector<vector<pair<int, int> > >;
const int INF = 2000000000ll;
const long double PI = acosl(-1);
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }



int power(int a, int b) {return b?power(a*a,b/2)*(b%2?a:1):1;}
int perm(int N, int K){
    int x=1;
    for(int i=1;i<=K;i++)
    {
        x=x*(N-i+1);
    }
    return x;
}
int factorial(int N) {return perm(N,N);}
int comb(int N,int K) {return factorial(N)/factorial(K)/factorial(N-K);}
int hcomb(int N, int K) {return comb(N+K-1, N-1);}

int dtoi(double d) {return round(d);}

int isprime(int n) {
    if (n < 2) return 0;
    for (int i = 2; i*i <= n; ++i) {
        if (n % i ==  0) return 0;
    }
    return 1;
}

int mystoi(string S){
    int num = 0;
    for (int i = (int) S.length()-1; i >= 0; i--){
        num += (S[i]-'0') * power(10,S.length()-1-i);
    }
    return num;
}

template<class T>
int SUM(T a) {
    int sum = 0;
    for(auto iter : a) sum += iter;
    return sum;
}

void solve(int A, int B, int C, int K);

signed main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    cout << fixed << setprecision(10);
    int A, B, C, K;
    cin >> A >> B >> C >> K;
    solve(A, B, C, K);
    return 0;
}

void solve(int A, int B, int C, int K) {
    int ans = 0;
    if (K <= A) ans = K;
    else if (A+1 <= K && K <= A+B) ans = A;
    else ans = A-(K-(A+B));
    pa;

}

/*
cd ~/Desktop/Programming/AtCoder/contest/abc167/abc167_b && sh test.sh
cd ~/Desktop/Programming/AtCoder/contest/abc167/abc167_b && sh etest.sh
cd ~/Desktop/Programming/AtCoder/contest/abc167/abc167_b && sh submit.sh
*/


Measured memory_bytes: 3644