#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll=long long;
using ld=long double;
ld pie=3.141592653589793;
ll inf=14449999999994;
ll mod=998244353 ;
int main(){
    ld t,s,d;
    cin >> t >> s >> d;
    ld tt=t+(d/s);
    if (t<18)
    {
        if (tt>18)
        {
            cout << setprecision(200) <<min(tt,(ld)30.0)-18.0 << endl;
        }else{
            cout << 0 << endl;
        }
    }else{
        cout << setprecision(200) << max((ld)0,min((ld)30.0,tt)-t) << endl;
    }
}