import java.util.*;
import java.util.Arrays;
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 sum =  0;
      for(int i=0;i<k;i++){
        if(a!=0){
          sum +=1;
          a--;
        }
        else if(b!=0){
          sum += 0;
          b--;
        }
        else{
          sum -=1;
          c--;
        }
      }
      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