#include <bits/stdc++.h>
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(i, n) for (int i = 0, i##_len = (n); i < i##_len; ++i)
#define REPR(i, n) for (int i = n; i >= 0; i--)
#define INF 1145141919
int dy[] = {0, 0, 1, -1};
int dx[] = {1, -1, 0, 0};
int ny, nx;
typedef long long int ll;
using namespace std;

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }

ll lcm(ll m, ll n)
{
    if ((0 == m) || (0 == n))
        return 0;
    return ((m / gcd(m, n)) * n);
}

ll llpow(ll x, ll y)
{
    ll ans = 1;
    REP(i, y)
    ans *= x;
    return ans;
}

int ctoi(char c)
{
    if (c >= '0' && c <= '9')
    {
        return c - '0';
    }
    return 0;
}
////////////////////////////////////////////////////////////////////
int main()
{
    int a,b,c,d; cin >> a >> b >> c >> d;
    int diff1 = b-a+1, diff2 = d-c+1;

    //cout << diff1 * diff2 << endl;
    int res=0;
    if((a<c and c<b and b<d) or (c<a and a<d and d<b)){
        res = max(b,c)-min(b,c)+1;
    }
    cout << diff1 * diff2 - res << endl;

    return 0;
}