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();
 	int count=0;
   
   	if(A>=K){
    count=K;
    }
   	else if(A<K&&A+B>=K){
    count=A;}
    else if(A+B<K){
    count=A-(K-A-B);}
 
  System.out.println(count);
 }
}

/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