import java.util.Scanner;

public class Main {
  public static void main(String[] args ) throws Exception {
    Scanner sc = new Scanner(System.in);
    int N = Integer.parseInt(sc.next());
    int N2 = Integer.parseInt(sc.next());
    int N3 = Integer.parseInt(sc.next());
    int N4 = Integer.parseInt(sc.next());
    if(N4-N <= 0) {
    	System.out.println(N4*1);
    } else if((N4-N) - N2 <= 0) {
    	System.out.println(N*1);
    } else {
    	System.out.println(N*1 + (-1 * ((N4-N) - N2)));
    }

  }
}
/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