import java.util.*;

public class Main{
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    long one = sc.nextLong();
    long zero = sc.nextLong();
    long minus = sc.nextLong();
    long card = sc.nextLong();
    long sum = 0;
    if(card <= one){
      System.out.println(card);
    }
    else{
      sum += one;
      card -= one;
      if(card > zero){
        card -= zero;
        for(int i = 0;i < card;i++){
          sum--;
        }
      }
      System.out.println(sum);
    }
  }
}
/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