1. Check condition 'First letter is not a vowel': Ivan (I - vowel), Nikolay (N - consonant), Semen (S - consonant), Igor (I - vowel).
2. Check condition 'Number of letters < 6': Ivan (4), Nikolay (7), Semen (5), Igor (4).
3. The name that satisfies both conditions is Semen (consonant first letter, 5 letters).