acm.sgu.ru 144

#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;

int main()
{
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
    int x,y;
    double z;

    cin>>x>>y>>z;
    double cnt = 0 ,win = 0;

    for (int i = 0; i <= 60*(y-x); i++)
    for (int j = i; j <= 60*(y-x); j++)
    {

        cnt++;
        if (abs(j-i) <= z) win++;
    }
    
    printf("%.7lf",win/cnt);
    
    return 0;
}