#include <bits/stdc++.h> using namespace std; typedef pair<int, int> pii; typedef long long ll; typedef vector<int> vi; #define pb push_back #define eb emplace_back #define mp make_pair #define fi first #define se second #define rep(i,n) rep2(i,0,n) #define rep2(i,m,n) for(int i=m;i<(n);i++) #define ALL(c) (c).begin(),(c).end() #define dump(x) cout << #x << " = " << (x) << endl constexpr ll TEN(int n) { return (n == 0) ? 1 : 10 * TEN(n-1); } template<class T, class U> ostream& operator<<(ostream& os, const pair<T, U>& p) { os<<"("<<p.first<<","<<p.second<<")"; return os; } template<class T> ostream& operator<<(ostream& os, const vector<T>& v) { os<<"{"; rep(i, v.size()) { if (i) os<<","; os<<v[i]; } os<<"}"; return os; } 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; }