#include <algorithm>
#include <cmath>
#include <cstdio>
#include <deque>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <string>
#include <vector>

using namespace std;

typedef long long ll;

#define MOD 1000000007

int main() {
  int a, b, c, d;
  cin >> a >> b >> c >> d;
  int ans = 0;
  for (int i = a; i <= b; ++i) {
    for (int j = c; j <= d; ++j) {
      if (i != j) ans++;
    }
  }
  cout << ans << endl;
  return 0;
}