import java.util.*;
public class Main { 
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        long o=0;
        long x=sc.nextLong();
        long y=sc.nextLong();
      	long z=sc.nextLong();
        long a=sc.nextLong();
      	if(a>x){
        	o+=x;  
          	a-=x;
          	a-=y;
          if(a>0){
          	o-=a;
          }
        }else{
          o=a;
        	      	
        }
        
      System.out.println(o);
    }
}  
/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