import java.util.*;

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 timesCounter=0;
      int maxCounter=0;
      
      for(int i=0; i<A; i++){
        if(timesCounter == K){
          break;
        }
        timesCounter++;
        maxCounter++;
      }
      
      for(int i=0; i<B; i++){
        if(timesCounter == K){
          break;
        }
        timesCounter++;
      }
      
      for(int i=0; i<C; i++){
        if(timesCounter == K){
          break;
        }
        timesCounter++;
        maxCounter--;
      }
      
      System.out.println(maxCounter);
    }
}
/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