#include <iostream>
#include <string>
using namespace std;
int main(){
	int A,B,C,D;
	cin>>A>>B>>C>>D;
	int y=D-C+1;
	int sum=0;
	for(int x=A;x<=B;x++){
		if((x>=C)&&(x<=D)){
			sum+=(y-1);
		}else{
			sum+=y;
		}
	}
	cout<<sum;
	return 0;
}