#include <bits/stdc++.h>
#define ft first
#define sc second
#define lb lower_bound
#define ub upper_bound
#define pb(t) push_back(t)
#define pt(num) cout << num << "\n"
#define chmax(a, b) (a<b ? a=b : 0)
#define chmin(a, b) (a>b ? a=b : 0)
#define max(a, b) ((a)>(b) ? (a):(b))
#define min(a, b) ((a)<(b) ? (a):(b))
#define moC(a, s, b) (a)=((a)s(b)+MOD)%MOD
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
typedef map<ll, ll> Map;
static const ll INF=1e18;
static const ll MAX=1e5+7;
static const ll MOD=1e9+7;



int main(void) {
    ll a, b, c, d;
    cin >> a >> b >> c >> d;
    
    if(b<c || d<a)
        pt((b-a+1)*(d-c+1));
    else
        pt((b-a+1)*(d-c+1)-(min(b, d)-min(a, b)+1));
}