#include <iostream>
using namespace std;
int main() {
    int n1, n2, t;
    float s = 0;
    cin >> n1 >> n2;
    t = n1 & 1 ? 1 : -1;
    for(float k = n1; k <= n2; k++) {
        s += t * (k - 1) / (k * k * k + k - 1);
        t = -t;
    }
    cout << s << endl;
    return 0;
}