#include<iostream>
using namespace std;
int main()
{
	int m,n,s=0;
	cin>>m>>n;
	if(n%2!=0)
		for(int i=n;i>m;i=i-2)
			s=s+i;
	else
		for(int i=n-1;i>m;i=i-2)
			s=s+i;
	cout<<s;
	return 0;
}