import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        
        int K = sc.nextInt();
        
        int ans;
        
        if(K<=A){
            ans = K;
        } else if(K<=A+B){
            ans = A;
        } else {
            ans = A - (K-(A+B));
        }
        
        
        System.out.println(ans);
    }
} import java.util.*; util.* java public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        
        int K = sc.nextInt();
        
        int ans;
        
        if(K<=A){
            ans = K;
        } else if(K<=A+B){
            ans = A;
        } else {
            ans = A - (K-(A+B));
        }
        
        
        System.out.println(ans);
    }
} Main public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        
        int K = sc.nextInt();
        
        int ans;
        
        if(K<=A){
            ans = K;
        } else if(K<=A+B){
            ans = A;
        } else {
            ans = A - (K-(A+B));
        }
        
        
        System.out.println(ans);
    } main {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        
        int K = sc.nextInt();
        
        int ans;
        
        if(K<=A){
            ans = K;
        } else if(K<=A+B){
            ans = A;
        } else {
            ans = A - (K-(A+B));
        }
        
        
        System.out.println(ans);
    } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int A = sc.nextInt(); A sc.nextInt() sc.nextInt sc int B = sc.nextInt(); B sc.nextInt() sc.nextInt sc int C = sc.nextInt(); C sc.nextInt() sc.nextInt sc int K = sc.nextInt(); K sc.nextInt() sc.nextInt sc int ans; ans if(K<=A){
            ans = K;
        } else if(K<=A+B){
            ans = A;
        } else {
            ans = A - (K-(A+B));
        } K<=A K A {
            ans = K;
        } ans = K ans K if(K<=A+B){
            ans = A;
        } else {
            ans = A - (K-(A+B));
        } K<=A+B K A+B A B {
            ans = A;
        } ans = A ans A {
            ans = A - (K-(A+B));
        } ans = A - (K-(A+B)) ans A - (K-(A+B)) A (K-(A+B)) K (A+B) A B System.out.println(ans) System.out.println System.out System System.out ans String[] args args public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        
        int K = sc.nextInt();
        
        int ans;
        
        if(K<=A){
            ans = K;
        } else if(K<=A+B){
            ans = A;
        } else {
            ans = A - (K-(A+B));
        }
        
        
        System.out.println(ans);
    }
} public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        
        int K = sc.nextInt();
        
        int ans;
        
        if(K<=A){
            ans = K;
        } else if(K<=A+B){
            ans = A;
        } else {
            ans = A - (K-(A+B));
        }
        
        
        System.out.println(ans);
    }
} Main
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        // Your code here!
        Scanner sc = new Scanner(System.in);
        
        int A = sc.nextInt();
        int B = sc.nextInt();
        int C = sc.nextInt();
        
        int K = sc.nextInt();
        
        int ans;
        
        if(K<=A){
            ans = K;
        } else if(K<=A+B){
            ans = A;
        } else {
            ans = A - (K-(A+B));
        }
        
        
        System.out.println(ans);
    }
}
