On using <code>x</code> operations of the second type and <code>y</code> operations of the first type, the stair <code>2^x - y</code> is reached.

Since first operations cannot be consecutive, there are exactly <code>x + 1</code> positions (before and after each power of 2) to perform the second operation.

Using combinatorics, we have ^{x + 1}C_y number of ways to select the positions of second operations.