Twist in towers of Hanoi problem

Towers of hanoi problem.

3 poles, N discs in decreasing size. (seen bottom to top, usual)

but there is one new condition. There are i number of discs (of same size i) at each ith position. A disc can be placed on top of same or bigger sized disc.

complete the below function to move all discs to another pole.

int getNumMoves(int N)
{

}

You will be given input number N, you have to output number of moves needed.

For ex:
For N = 3 NumMoves is 17
For N = 4 NumMoves is 49