import java.util.Scanner;

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();
	if(k>0){
	    if(k<a){
		System.out.println(k);
	    }else if(k<=a+b){
		System.out.println(a);
	    }else{
		System.out.println(2*a+b-k);
	    }
	}else{
	    System.out.println(0);
	}
      }
} import java.util.Scanner; Scanner java.util 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();
	if(k>0){
	    if(k<a){
		System.out.println(k);
	    }else if(k<=a+b){
		System.out.println(a);
	    }else{
		System.out.println(2*a+b-k);
	    }
	}else{
	    System.out.println(0);
	}
      }
} 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();
	if(k>0){
	    if(k<a){
		System.out.println(k);
	    }else if(k<=a+b){
		System.out.println(a);
	    }else{
		System.out.println(2*a+b-k);
	    }
	}else{
	    System.out.println(0);
	}
      } main {
	Scanner sc = new Scanner(System.in);
	int a = sc.nextInt();
	int b = sc.nextInt();
	int c = sc.nextInt();
	int k = sc.nextInt();
	if(k>0){
	    if(k<a){
		System.out.println(k);
	    }else if(k<=a+b){
		System.out.println(a);
	    }else{
		System.out.println(2*a+b-k);
	    }
	}else{
	    System.out.println(0);
	}
      } 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 if(k>0){
	    if(k<a){
		System.out.println(k);
	    }else if(k<=a+b){
		System.out.println(a);
	    }else{
		System.out.println(2*a+b-k);
	    }
	}else{
	    System.out.println(0);
	} k>0 k 0 {
	    if(k<a){
		System.out.println(k);
	    }else if(k<=a+b){
		System.out.println(a);
	    }else{
		System.out.println(2*a+b-k);
	    }
	} if(k<a){
		System.out.println(k);
	    }else if(k<=a+b){
		System.out.println(a);
	    }else{
		System.out.println(2*a+b-k);
	    } k<a k a {
		System.out.println(k);
	    } System.out.println(k) System.out.println System.out System System.out k if(k<=a+b){
		System.out.println(a);
	    }else{
		System.out.println(2*a+b-k);
	    } k<=a+b k a+b a b {
		System.out.println(a);
	    } System.out.println(a) System.out.println System.out System System.out a {
		System.out.println(2*a+b-k);
	    } System.out.println(2*a+b-k) System.out.println System.out System System.out 2*a+b-k 2*a+b 2*a 2 a b k {
	    System.out.println(0);
	} System.out.println(0) System.out.println System.out System System.out 0 String args[] args
import java.util.Scanner;

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();
	if(k>0){
	    if(k<a){
		System.out.println(k);
	    }else if(k<=a+b){
		System.out.println(a);
	    }else{
		System.out.println(2*a+b-k);
	    }
	}else{
	    System.out.println(0);
	}
      }
}
