runtime of program
auto start = chrono::high_resolution_clock::now();
foo(bar);
auto end = chrono::high_resolution_clock::now();
auto elapsed = chrono::duration_cast<chrono::milliseconds>(end - start);
cerr << " elapsed " << elapsed.count() << " milliseconds \n";
precision of doubles in cout
cout.precision(17)
flush cout before reading from cin
cin.tie(0)
don`t synchronize cout and printf
ios::sync_with_stdio(0);
unlimited stack depth
ulimit -s unlimited