#include <iostream>
#include <fstream>
#include <string>
#include <random>
using namespace std;

double next_random() {
    static random_device dev;
    static default_random_engine engine {dev()};
    static uniform_real_distribution<double> dist {0, 1};
    return dist(engine);
}

string random_line_from_file(istream& file) {
    // This function selects a line from a file by reading
    // the file line by line, so the file is never in memory
    // as a whole. Still, the chance a line is being picked
    // is equal for every line.
    string line;
    string result;
    for (int nr = 1; getline(file, line); ++nr) {
        if (next_random() * nr < 1.0) {
            result = line;
        }
    }
    return result;
}

int main(int argc, const char * argv[]) {
    if (argc > 1) {
        ifstream infile {argv[1]};
        cout << random_line_from_file(infile) << '\n';
    } else {
        cout << random_line_from_file(cin) << '\n';
    }
}