#include<stdio.h>
#include<stdlib.h>

#define STR_LENGTH 16
#define NUM_INPUT 4

void str_cut(char*, unsigned char*);

int main(void){
  unsigned char diff = 0;
  unsigned char same = 0;
  char str[STR_LENGTH];
  unsigned char input[NUM_INPUT] = {0};
  unsigned int ans = 0;

  fgets(str, sizeof(str), stdin);
  str_cut(str, input);

  if((input[3] < input[0]) || (input[1] < input[2])) {
    ans = (input[1] - input[0] + 1) * (input[3] - input[2] + 1);
  } else {
    diff = abs(input[2] - input[0]) + abs(input[3] - input[1]);
    same = ((input[1] - input[0] + 1) + (input[3] - input[2] + 1 - diff)) / 2;
    ans = (input[1] - input[0] + 1) * (input[3] - input[2] + 1) - same;
  }

  printf("%d\n", ans);
  return 0;
}

/*
関数概要：渡された文字列を空白で分離し、数値に変換する
第1引数：文字列
第2引数：文字列から数値に変換して格納する配列
*/
void str_cut(char* str, unsigned char* input){
  int start_str = 0;
  int num = 0; //配列NO
  int i = 0;

  while('\0' != str[i]){
    if(' ' == str[i]){
      str[i] = '\0';
      input[num] = (unsigned char) atoi(&str[start_str]);
      ++num;
      ++i;
      start_str = i;
    }
    ++i;
  }

  input[num] = (unsigned char) atoi(&str[start_str]);

  return;
}