#include <bits/stdc++.h>
using namespace std;
#if __has_include(<atcoder/all>)
#include <atcoder/all>
using namespace atcoder;
typedef modint998244353 mint;
#endif
typedef long long  ll;
typedef pair<int,int> pii; typedef pair<ll,ll> pll;
const int INF = 1e9; const long long LINF = 1e18;
const int MOD = 998244353; const int MOD1 = 1e9+7;
#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define all(x) x.begin(), x.end()
template<typename T> bool chmin(T& a, T b){if(a > b){a = b; return true;} return false;}
template<typename T> bool chmax(T& a, T b){if(a < b){a = b; return true;} return false;}
void yorn(bool ans){cout << (ans?"Yes":"No") << endl; return;}

// しばらくはsize_tをキャストに任せてみようかな
int main(){
    double t,s,d; cin >> t >> s >> d;
    double T = d/s;

    double now = 0;
    double ans = 0;
    while(now<T){
        //cout << now << endl;
        if(6<=t && t<18){
            now += min(T-now,18-t);
            t = 18;
        }
        else{
            double pre = now;
            if(18<=t&&t<24) now += min(T-now,30-t);
            else now += min(T-now,6-t);
            ans += now-pre;
            t = 6;
        }
    }
    cout << fixed << setprecision(10) << ans << endl;
}