import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] str = br.readLine().split(" ");
        int a = Integer.parseInt(str[0]);
        int b = Integer.parseInt(str[1]);
        int c = Integer.parseInt(str[2]);
        int d = Integer.parseInt(str[3]);

        int[] time = new int[101];
        time[a] += 1;
        time[b] += -1;
        time[c] += 1;
        time[d] += -1;
        for (int i = 1; i < 101; i++) time[i] += time[i - 1];

        int cnt = 0;
        for (int i = 0; i < 101; i++) if (time[i] == 2) cnt++;

        System.out.println(cnt);
    }
}

/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