#include <stdio.h>

main(){
	int range[4],stock[4];
	int count;
	int k,j;
	char buf[100];

	//データの入力
	fgets(buf,sizeof(buf),stdin);
	sscanf(buf,"%d %d %d %d",&range[0],&range[1],&range[2],&range[3]);
	for(k=0;k<4;k++){
		stock[k]=range[k];
	}

	while(range[0]<=range[1]){
		while(range[2]<=range[3]){
			if(range[0]!=range[2]){
				count++;
			}
		range[2]++;
		}
		range[2]=stock[2];
		range[0]++;
	}

	printf("%d\n",count);
	return(0);
}