double averageWaitingTime(vector<vector<int>>& customers) {

double sum = 0;

int idle = 0;

for (int i = 0; i < customers.size(); i++) {

int arrival_i = customers[i][0];

int time_i = customers[i][1];

idle = max(idle, arrival_i) + time_i;

sum += idle - arrival_i;

}

return sum / customers.size();

}