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 d = sc.nextInt();
    int count = 0;
    for (int i = a; i <= b; i++) {
      for (int j = c; j <= d; j++) {
        if (i == j) {
          count++;
        }
      }
    }
    if (count == 0) {
      System.out.println(count);
    } else {
      System.out.println(count - 1);
    }
  }
} import java.util.*; util.* java 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 d = sc.nextInt();
    int count = 0;
    for (int i = a; i <= b; i++) {
      for (int j = c; j <= d; j++) {
        if (i == j) {
          count++;
        }
      }
    }
    if (count == 0) {
      System.out.println(count);
    } else {
      System.out.println(count - 1);
    }
  }
} 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 d = sc.nextInt();
    int count = 0;
    for (int i = a; i <= b; i++) {
      for (int j = c; j <= d; j++) {
        if (i == j) {
          count++;
        }
      }
    }
    if (count == 0) {
      System.out.println(count);
    } else {
      System.out.println(count - 1);
    }
  } main {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    int c = sc.nextInt();
    int d = sc.nextInt();
    int count = 0;
    for (int i = a; i <= b; i++) {
      for (int j = c; j <= d; j++) {
        if (i == j) {
          count++;
        }
      }
    }
    if (count == 0) {
      System.out.println(count);
    } else {
      System.out.println(count - 1);
    }
  } Scanner sc = new Scanner(System.in); sc new Scanner(System.in) int a = sc.nextInt(); a sc.nextInt() sc.nextInt sc int b = sc.nextInt(); b sc.nextInt() sc.nextInt sc int c = sc.nextInt(); c sc.nextInt() sc.nextInt sc int d = sc.nextInt(); d sc.nextInt() sc.nextInt sc int count = 0; count 0 for (int i = a; i <= b; i++) {
      for (int j = c; j <= d; j++) {
        if (i == j) {
          count++;
        }
      }
    } int i = a; int i = a; i a i <= b i b i++ i++ i {
      for (int j = c; j <= d; j++) {
        if (i == j) {
          count++;
        }
      }
    } {
      for (int j = c; j <= d; j++) {
        if (i == j) {
          count++;
        }
      }
    } for (int j = c; j <= d; j++) {
        if (i == j) {
          count++;
        }
      } int j = c; int j = c; j c j <= d j d j++ j++ j {
        if (i == j) {
          count++;
        }
      } {
        if (i == j) {
          count++;
        }
      } if (i == j) {
          count++;
        } i == j i j {
          count++;
        } count++ count if (count == 0) {
      System.out.println(count);
    } else {
      System.out.println(count - 1);
    } count == 0 count 0 {
      System.out.println(count);
    } System.out.println(count) System.out.println System.out System System.out count {
      System.out.println(count - 1);
    } System.out.println(count - 1) System.out.println System.out System System.out count - 1 count 1 String[] args args
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 d = sc.nextInt();
    int count = 0;
    for (int i = a; i <= b; i++) {
      for (int j = c; j <= d; j++) {
        if (i == j) {
          count++;
        }
      }
    }
    if (count == 0) {
      System.out.println(count);
    } else {
      System.out.println(count - 1);
    }
  }
}