import java.util.*;
class Main{
public static void main(String[]args){
Scanner sc=new Scanner(System.in);
  int a=sc.nextInt(),b=sc.nextInt(),c=sc.nextInt(),k=sc.nextInt();
  if(a>k){
    System.out.println(k);
  }else{
      k-=a;
      if(b>k){
          System.out.println(a);
      }else{
          k-=b;
          System.out.println(a-k);
      }
  }
}
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz