No closed form equation is known, as discussed in A061418.

For fun, here's a bitwise solution:

m = 2
tot = 2
for _ in range(1, input()):
    m += m>>1
    tot += m
print tot

Here's how it works:

1. floor(3x / 2)
2. floor(2x/2 + x/2)
3. floor(x + x/2)
4. x + floor(x/2) --->{since x is int, see here}
5. x + x>>1 -------->{bitwise shift right x by k = floor(x/2^k}

Method taken from here