import java.util.*;
import java.util.Arrays;
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 sum =  0;
      for(int i=0;i<k;i++){
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      }
      System.out.println(sum);

    
     

      
    }
} import java.util.*; util.* java import java.util.Arrays; Arrays 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 sum =  0;
      for(int i=0;i<k;i++){
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      }
      System.out.println(sum);

    
     

      
    }
} 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 sum =  0;
      for(int i=0;i<k;i++){
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      }
      System.out.println(sum);

    
     

      
    } main {
      Scanner sc = new Scanner(System.in);
      int a = sc.nextInt();
      int b = sc.nextInt();
      int c = sc.nextInt();
      int k = sc.nextInt();
      int sum =  0;
      for(int i=0;i<k;i++){
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      }
      System.out.println(sum);

    
     

      
    } 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 sum =  0; sum 0 for(int i=0;i<k;i++){
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      } int i=0; int i=0; i 0 i<k i k i++ i++ i {
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      } {
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      } if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        } a!=0 a 0 {
          sum +=1;
          a--;
        } sum +=1 sum 1 a-- a if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        } b!=0 b 0 {
          sum += 0;
          b--;
        } sum += 0 sum 0 b-- b {
          sum -=1;
          c--;
        } sum -=1 sum 1 c-- c System.out.println(sum) System.out.println System.out System System.out sum 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 sum =  0;
      for(int i=0;i<k;i++){
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      }
      System.out.println(sum);

    
     

      
    }
} 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 sum =  0;
      for(int i=0;i<k;i++){
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      }
      System.out.println(sum);

    
     

      
    }
} Main
import java.util.*;
import java.util.Arrays;
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 sum =  0;
      for(int i=0;i<k;i++){
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      }
      System.out.println(sum);

    
     

      
    }
}