Some girls are evil as described in the equation and some are not that way at all. Spend some time with the girl with whom you wish have a long term relationship, may be at least about 6 months, by then talk and spend time often with her. She will reveal lot about herself or at least you will get much idea if she is the right girl for the rest of your life. Play a dumb guy, if she attempts to take advantage of you then you have to know she is not the one for you. Play a really broke person, if she leaves you , you are safe because she will leave you anyway when you actually get broke. We never know when we will end up being broke or hit a really great jackpot. Care about her sincerely, if she does not appreciate your sincerity, she is not the one for you. Try to be honest to her, is she is not equally honest to you, she is not the one for you.