#define _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
using namespace std;
#define rep(i, n) for(int i=0; i<(n); ++i)
#define FOR(i, m, n) for(int i=(m); i<(n); ++i)
#define sz(x) ((int)(x).size())
#define all(x) (x).begin(),(x).end()
#define mp make_pair
#define pb push_back
#define Cout(x) cout << (x) << endl
#define Cout2(x, y) cout << (x) << " " << (y) << endl
#define dump(x) cout << #x << " = " << (x) << endl;
#define forauto(i, a) for(auto i : a)cout << i << " "; puts("");
typedef long long LL;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<string> VS;
typedef vector<bool> VB;
typedef vector<vector<int>> VVI;
typedef pair<int, int> PII;
const int inf = 1e9;
const double pi = acos(-1.0);

int main() {
	int a, b, c, d; cin >> a >> b >> c >> d;
	int ans = 0;
	FOR(i, a, b + 1) {
		FOR(j, c, d + 1) {
			if (i != j)ans++;
		}
	}
	Cout(ans);
}