import java.util.HashMap;
import java.util.Map;

class Scratch {
    public static void main(String[] args) {
        long number = 600851475143L;
//        long number = 13195;
        long result = findLargestPrimeFactor(number);
        System.out.println(result);
    }

    private static long findLargestPrimeFactor(long number) {
        long squareRoot = (long)Math.ceil(Math.sqrt(number));
        Map<Long, Boolean> eratosthenesReturn = eratosthenesSieve(squareRoot);
        long max = Integer.MIN_VALUE;
        for (long l : eratosthenesReturn.keySet()) {
            if (l > max && !eratosthenesReturn.get(l) && (number % l == 0))
                max = l;
        }
        return max;
    }

    private static Map<Long, Boolean> eratosthenesSieve(long max) {
        // boolean = is composite. false if prime
        Map<Long, Boolean> sieve = new HashMap<>();
//        sieve.put(2L, false);
//        sieve.put(3L, false);
        for (long i = 2; i <= max; i++) {
            sieve.put(i, false);
        }
        for (long i = 2; i < max; i++) {
            if (!sieve.get(i)) {
                for (long j = i * 2; j < max; j += i) {
                    sieve.put(j, true);
                }
            }
        }
        return sieve;
    }
}