I'm filing documents for marriage together with my American husband. I've got two children who live in Russia but cannot find the men with whom I had short relationships. From them I had two children from 2 different men in Russia and raised the children myself. I was always a single mother in that country(and I can prove it). It's impossible to find those men now. My children are 21 (daughter) and 16(son). How can I explain all that while filing I-485 and I-130?