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 D = sc.nextInt();
    sc.close();
    
    int result = 0;
    
     for(int i = 0; i < D; i++){
       if (A > 0){
         result += 1;
         A -= 1;
       } else if ( B > 0){
         B -= 1;
       } else {
         result -= 1;
         C -= 1;
       }
     }
    System.out.println(result);
  } 
}

/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