#include <bits/stdc++.h>

#define fst(t) std::get<0>(t)
#define snd(t) std::get<1>(t)
#define thd(t) std::get<2>(t)
#define unless(p) if(!(p))
#define until(p) while(!(p))

using ll = std::int64_t;
using P = std::tuple<int,int>;

int A, B, C, D;

int main(){
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    std::cin >> A >> B >> C >> D;

    int cnt = 0;
    for(int i=A;i<=B;++i){
        for(int j=C;j<=D;++j){
            if(i != j){
                cnt += 1;
            }
        }
    }

    std::cout << cnt << std::endl;
}