#include <bits/stdc++.h> #define int long long #define ll long long #define rep(i,n) for(int i=0;i<n;i++) #define INF LLONG_MAX/3 #define eps LDBL_EPSILON #define moder (1000000007) #define pie acos(-1) #define P std::pair<int,int> #define prique priority_queue #define ggr getchar();getchar();return 0; using namespace std; int a,b,c,d; signed main(){ 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)ans++; } } cout<<ans<<endl; return 0; }