#include <iostream> #include <string> #include <set> #include <map> #include <vector> #include <queue> #include <algorithm> #include <bitset> #include <cstdlib> #include <cmath> #define INF 100000000 #define ll long long int using namespace std; int main(){ int A,B,C,D; cin>>A>>B>>C>>D; int pre = (B-A+1) * (D-C+1); int cnt=0; vector <int> box; box.resize(200,0); for(int i=A;i<=B;i++) box[i]++; for(int i=C;i<=D;i++) box[i]++; for(int i=0;i<=120;i++){ if(box[i]==2) cnt++; } cout<<pre-cnt<<endl; return 0; }