#include<bits/stdc++.h>
using namespace std;
int main()
{
    int A,B,C,D; cin >> A >> B >> C >> D;
    if(A > D || C > B)
    {
        cout << (B - A + 1)*(D - C + 1) << endl;
    }
    else if(A <= C && B >= C && B <= D)
    {
        cout << (B - A + 1)*(D - C + 1) - (B - C + 1) << endl;
    }
    else if(C <= A && A <= D && D <= B)
    {
        cout << (B - A + 1)*(D - C + 1) - (D - A + 1) << endl;
    }
    else if(A <= C && C <= D && D <= B)
    {
        cout << (B - A + 1)*(D - C + 1) - (D - C + 1) << endl;
    }
    else
    {
        cout << (B - A + 1)*(D - C + 1) - (B - A + 1) << endl;
    }
}