#include "bits/stdc++.h"

using namespace std;

#define ll long long
#define vi  vector<int>
#define vvi vector<vi>
#define pi  pair<int,int>
#define mp  make_pair
#define pb  push_back
#define MOD 1e9 + 7
#define PAI  3.1415
#define all(x) (x).begin(),(x).end()
#define chmax(x,y) x = max(x,y)
#define chmin(x,y) x = min(x,y)
#define pr(x) cout << x << endl
#define Endl endl
#define rep(i,n) for(int i = 0 ; i < n; i++)

const int dx[4] = {1,0,-1,0};
const int dy[4] = {0,1,0,-1};
const int ddx[8] = {-1,0,1,-1,1,-1,0,1};
const int ddy[8] = {-1,-1,-1,0,0,1,1,1};
const int inf = 99999999;
const ll linf = 1LL << 62;

ll gcd(ll a,ll b){
  if(a < b)swap(a , b);
  if(a % b == 0) return b;
  else gcd(b, a%b);
}

ll lcm(ll a,ll b){
  if(a < b)swap(a , b);
  return (a/gcd(a , b))*b;
}

int main(){

  int a, b, c, d; cin >> a >> b >> c >> d;

  vector<pi > v;

  for(int i = a; i <= b; i++)
  {
    for(int j = c; j <= d; j++)
    {
      v.pb(mp(i,j));
    }
  }
  
  int ans = 0;

  for(int i = 0; i < v.size(); i++)
  {
    if(v[i].first != v[i].second)
      ans ++; 
  }
  
  pr(ans);

  return 0;
}