#include<iostream>
using namespace std;
int main ()
{
int a, b, s1, s2;
cin >> a >> b;
if ((a % 2 == 0) and (b % 2 == 0)) {
    s1 = (a + b)/2 * ((b-a)/2 + 1);
    s2 = (a+1 + b-1)/2 * ((b-a-2)/2+1);
}
if ((a % 2 != 0) and (b % 2 != 0)) {
    s2 = (a + b)/2 * ((b-a)/2 + 1);
    s1 = (a+1 + b-1)/2 * ((b-a-2)/2+1);
}
if ((a % 2 != 0) and (b % 2 == 0)) {
    s1 = (a + b+1)/2 * ((b-a-1)/2 + 1);
    s2 = (a + b-1)/2 * ((b-a-1)/2+1);
}
if ((a % 2 == 0) and (b % 2 != 0)) {
    s2 = (a + b+1)/2 * ((b-a-1)/2 + 1);
    s1 = (a + b-1)/2 * ((b-a-1)/2+1);
}
cout << s1 - s2;
return 0;
}