import java.util.*;
 
public class Main{
  public static void main(String[]args){
    Scanner sc =  new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int c = sc.nextInt();
    int k = sc.nextInt();
    
    int ans = 0;
    ans += Math.min(a,k);
    k -= a;
    k -= b;
    if(k > 0){
      ans -= Math.min(c,k);
    }
    System.out.println(ans);
  }
} import java.util.*; util.* java public class Main{
  public static void main(String[]args){
    Scanner sc =  new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int c = sc.nextInt();
    int k = sc.nextInt();
    
    int ans = 0;
    ans += Math.min(a,k);
    k -= a;
    k -= b;
    if(k > 0){
      ans -= Math.min(c,k);
    }
    System.out.println(ans);
  }
} Main public static void main(String[]args){
    Scanner sc =  new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int c = sc.nextInt();
    int k = sc.nextInt();
    
    int ans = 0;
    ans += Math.min(a,k);
    k -= a;
    k -= b;
    if(k > 0){
      ans -= Math.min(c,k);
    }
    System.out.println(ans);
  } main {
    Scanner sc =  new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int c = sc.nextInt();
    int k = sc.nextInt();
    
    int ans = 0;
    ans += Math.min(a,k);
    k -= a;
    k -= b;
    if(k > 0){
      ans -= Math.min(c,k);
    }
    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 = 0; ans 0 ans += Math.min(a,k) ans Math.min(a,k) Math.min Math a k k -= a k a k -= b k b if(k > 0){
      ans -= Math.min(c,k);
    } k > 0 k 0 {
      ans -= Math.min(c,k);
    } ans -= Math.min(c,k) ans Math.min(c,k) Math.min Math c k 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){
    Scanner sc =  new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int c = sc.nextInt();
    int k = sc.nextInt();
    
    int ans = 0;
    ans += Math.min(a,k);
    k -= a;
    k -= b;
    if(k > 0){
      ans -= Math.min(c,k);
    }
    System.out.println(ans);
  }
} public class Main{
  public static void main(String[]args){
    Scanner sc =  new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int c = sc.nextInt();
    int k = sc.nextInt();
    
    int ans = 0;
    ans += Math.min(a,k);
    k -= a;
    k -= b;
    if(k > 0){
      ans -= Math.min(c,k);
    }
    System.out.println(ans);
  }
} Main
import java.util.*;
 
public class Main{
  public static void main(String[]args){
    Scanner sc =  new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int c = sc.nextInt();
    int k = sc.nextInt();
    
    int ans = 0;
    ans += Math.min(a,k);
    k -= a;
    k -= b;
    if(k > 0){
      ans -= Math.min(c,k);
    }
    System.out.println(ans);
  }
}