X is number of coin flips needed to get m tails OR m heads. What is the cumulative distribution function of x?

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So far, I know $$P(x=k)={{k-1} \choose m-1}*2^{1-k}$$ and $$\sum_{k=m}^{2m-1}{ {k-1} \choose m-1}*2^{1-k} = 1$$