Independent probabilities question

Dear All,

I am new to probability theory and it would be a great help if you can help me answering this question.

There are N balls. There is a circle in the space. The probability of each ball to fall in the circle is p. What is the probability that there are *at least* k out of N balls that fall in the circle? I came up with the probability of at least one out of N balls falling in the circle and that is ( 1 - (1 - p)^N). Can someone please guide me how to do for k.

Thanks ,

Cheema