class UndergroundSystem {

// id => entryStation

string checkInName[1000001];

// id => entryTime

int checkInTime[1000001];

// id => {numberOfTravels, overallTime}

unordered_map<string, pair<int, int>> avgTimesMap;

UndergroundSystem() {

// cout << "I am a constructor and I am as useless as \"ueue\" in \"queue\"\n";

}

void checkIn(int id, string stationName, int t) {

checkInName[id] = stationName;

checkInTime[id] = -t;

}

void checkOut(int id, string stationName, int t) {

// retrieving matching checkin data

int travelTime = checkInTime[id] + t;

// computing travelName id

string travelName = checkInName[id] + stationName;

// updating data on the current travel

auto currTravel = avgTimesMap.find(travelName);

if (currTravel == end(avgTimesMap)) avgTimesMap[travelName] = {1, travelTime};

else {

currTravel->second.first++;

currTravel->second.second += travelTime;

}

}

double getAverageTime(string startStation, string endStation) {

// computing travelName id

string travelName = startStation + endStation;

// retrieving data on current travel

auto currTravel = avgTimesMap[travelName];

// returning the average time

return currTravel.second / (double)currTravel.first;

}

};