you gotta guess well
We reverse the binary and find that it reads the flag, reads our input, compares them, and tells us if we're right.
But wait: if we're wrong, it tells us what our flag was, and it uses printf for that, with our input as a format string.
Furthermore, the flag is read into a buffer on the stack.
Hence, we can leak the flag by making it print it out.
Two details: this is 64-bit, so we need to use the ll modifier to print out 64-bit integers;
and $ lets us skip to later arguments quickly.
In any case, we end up with a hexdump of the flag.