import java.util.Scanner;

public class Main {

    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        int A = Integer.parseInt(sc.next());
        int B = Integer.parseInt(sc.next());
        int C = Integer.parseInt(sc.next());
        int K = Integer.parseInt(sc.next());
        int max=0;


        if( K < A){
            max = K;
            K=0;
        }else{
            for(int i=0; i<A; i++){
                max++;
                K--;
            }
        }
        if(K > 0){
            K-=B;
        }
        if(K > 0){
            while(K > 0){
                K--;
                max--;
            }
        }
        A=C-1;

        System.out.println(max);

        sc.close();
    }
    
}
/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