#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using vi = vector<int>;

#define rep1(a)          for(int i = 0; i < ll(a); i++)
#define rep2(i, a)       for(int i = 0; i < ll(a); i++)
#define rep3(i, a, b)    for(int i = ll(a); i < ll(b); i++)
#define rep4(i, a, b, c) for(int i = ll(a); i < ll(b); i += ll(c))
#define overload4(a, b, c, d, e, ...) e
#define rep(...) overload4(__VA_ARGS__, rep4, rep3, rep2, rep1)(__VA_ARGS__)
#define all(x)  begin(x),end(x)
#define rall(x) rbegin(x),rend(x)

int main() {
	std::ios_base::sync_with_stdio(false);
	std::cin.tie(nullptr);
	int a, b, c, d;
	cin >> a >> b >> c >> d;
	int ans = 0;
	rep(x, a, b + 1) rep(y, c, d + 1) {
		if (x != y) ans++;
	}
	cout << ans << endl;
}