#include<iostream>
#include<vector>
#include<algorithm>
#include<queue>
#include<stack>
#include<cstring>
#include<utility>
#include<cmath>
#include<assert.h>
#include<set>
#include<map>
#include<unordered_set>
#include<unordered_map>
#include<complex>

#define int long long
using namespace std;
#define rep(i, n) for(int i=0;i<(n);++i)
typedef pair<int, int> pii;
const int INF = 1l << 60;
#define u_b upper_bound
#define l_b lower_bound


signed main() {
    int a, b, c, d;
    cin >> a >> b >> c >> d;
    int ans = 0;
    for (int i = a; i <= b; ++i) {
        for (int j = c; j <= d; ++j) {
            ans += i != j;
        }
    }
    cout << ans << endl;
    return 0;
}