#include<bits/stdc++.h>
using namespace std;

int main (){
    double a, b, c;
    cin >> a >> b >> c;
    double sum;

    sum += (a + 1) /2 * a;

    for(int i = 1; i <= b; i++)
        sum += i*i;

    for(double i = 1; i <= c; i++)
        sum += 1.0/i;

    printf("%.2f\n", sum);

    return 0;
}