The leader of an IMO team chooses positive integers $n$ and $k$ with $n>k$, and announces them to the deputy leader and a contestant. The leader then secretly tells the deputy leader an $n$-digit binary string, and the deputy leader writes down all $n$-digit binary strings which differ from the leader's in exactly $k$ positions. (For example, if $n=3$ and $k=1$, and if the leader chooses $101$, the deputy leader would write down $001,111$and $100$.) The contestant is allowed to look at the strings written by the deputy leader and guess the leader's string. What is the minimum number of guesses (in terms of $n$ and $k)$ needed to guarantee the correct answer? Please give the sum of guesses when $n = 6$ and $k = 3$. 

Let's think step by step and output the final answer within \boxed{}.
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