#include<bits/stdc++.h>
using namespace std;
#include<atcoder/all>
using namespace atcoder;
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define rep1(i,n) for(int i=1;i<=(int)n;i++)
using ll = long long;
using P = pair<int, int>;
using Pl = pair<ll, ll>;
using vi = vector<int>;
using vvi = vector<vi>;
using vd = vector<double>;
using vs = vector<string>;
using vb = vector<bool>;
using vl = vector<long long>;
using vvl = vector<vl>;
#define inf 2147483646//int max
#define linf 900000000000000000
const double PI = 3.1415926535897932384626433832795028841971;
template<typename T>inline bool chmax(T& a, T b) { return ((a < b) ? (a = b, true) : (false)); }
template<typename T>inline bool chmin(T& a, T b) { return ((a > b) ? (a = b, true) : (false)); }
long long llceil(long long a, long long b) { if (a % b == 0) { return a / b; }return (a / b) + 1; }
#define yes cout<<"Yes"<<endl;
#define no cout<<"No"<<endl;


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)continue;
            ans++;
        }
    }
    cout << ans << endl;
    return 0;
}