Yalynky

Just before the New Year, an alley in the city park had N fir trees planted in a row, with equal distances between each neighboring tree. After the New Year holidays, only M fir trees remained, still maintaining the equal distances between them.

Since there is no way to catch the thieves, we need to determine how many different ways the trees could have been cut down.

Input Data

You will be given two natural numbers, N and M (1 ≤ M, N ≤ 1000).

Output Data

Output the number of possible ways to cut down the trees.

Examples