#include "bits/stdc++.h" using namespace std; using Graph = vector>; typedef long long ll; typedef pair P; #define rep(i,n) for (ll i = 0; i < (n); ++i) #define all(x) (x).begin(), (x).end() #define rall(x) (x).rbegin(), (x).rend() const long long mod = 1e9 + 7; ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; } ll lcm(ll a, ll b) { return a / gcd(a, b) * b; } int main() { ll a,b,c,d; cin >> a >> b >> c >> d; ll ans = 0; for (int i = a; i <= b; i++) { for (int j = c; j <= d; j++) { if (i != j) { ans++; } } } cout << ans << endl; }