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So, it is rather interesting-the were 12 infantery and 3 cavalery divisions in 51st Special soviet army-but only one(!) was in RH at the Perekop because its commander was afraid of enemy's landing. If the are more soviet troops at the Perekop, 51st army would be able to stop 11th german army at the gates of the Crimea. So there are two variants:
1) More german troops took pert in battle for Rostov-na-Donu(maybe soviet wouldnt kick them out)
2) No Operation Blau-german spent more time in Crimea spring-summer 1942