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 K = sc.nextInt();
      int score = 0;
      while(K > 0){
        if(A > 0){
          A--;
          K--;
          score++;
        }
        else if(B > 0){
          B--;
          K--;
        }
        else if(C > 0){
          C--;
          K--;
          score--;
        }
      }
      System.out.println(score);
	
	}
}
/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