#include<iostream> #include<algorithm> #include<string> #include<vector> #include<cmath> #include<map> #include<iomanip> #include<queue> #include<stack> #include<time.h> #define ll long long #define rep(i,n) for(int i=0;i<n;i++) using namespace std; signed main() { ll a, b, c, d; cin >> a >> b >> c >> d; if (a > c) { swap(a, c); swap(b, d); } else if (a == c && b > d) { swap(b, d); } if (b < c)cout << (b - a + 1)*(d - c + 1) << endl; else if (a<c&&b>d)cout << (b - a + 1)*(d - c + 1) - (d - c + 1) << endl; else cout << (b - a + 1)*(d - c + 1) - (b - c + 1) << endl; getchar(); getchar(); return 0; }