import java.util.*;


public class Main{
    public static void main(String[] args) throws Exception{
        
        Scanner sc = new Scanner(System.in);
        
        int a = sc.nextInt();
        int b = sc.nextInt();
        int c = sc.nextInt();
        int k = sc.nextInt();//3
        int sum_number = 0;

        int[] numbers = {1,0,-1};
        int[] cards = {a,b,c}; //2,1,1

        for(int i = 0; i<=2; i++){
            if(cards[i]<=k){
                sum_number = sum_number + (numbers[i] * cards[i]);
                k = k - cards[i];

            }else{
                sum_number = sum_number + (numbers[i] * k);
                break;
            }
        }
        System.out.println(sum_number);
        return;
    }    
}
/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