#include<iostream>


int main(void){

	int A,B,C,D;

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

	//WA対応
	int xx=(B>D?D:B)-(A>C?A:C)+1;
	if(xx<0){
		xx=0;
	}

	std::cout<<(B-A+1)*(D-C+1)-xx<<std::endl;

	return 0;
}