#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define pb push_back
#define fi first
#define se second
typedef pair<ll,ll> P;
#define Mod 1000000007
using VP = vector<P>; using VVP = vector<VP>;
using VI = vector<ll>; using VVI = vector<VI>; using VVVI = vector<VVI>;

int main(){
    ll i,j;
    ll a,b,c,d;
    cin>>a>>b>>c>>d;
    ll ans=(b-a+1)*(d-c+1);
    ll m=max(a,c);
    ll M=min(b,d);
    if(m<=M) ans-=M-m+1;

    cout<<ans<<endl;
    
    return 0;
}