Count Grid Paths

Start at the top-left corner of a grid with rows by columns cells. Move along cell edges only, one edge right or one edge down at a time, until reaching the bottom-right corner. Count distinct move sequences. The grid has no obstacles.

A grid with 2 rows and 3 columns of cells needs exactly 2 down moves and 3 right moves.

Implement lattice_paths(rows, columns).

Input

rows: The number of rows of cells, not vertices.

columns: The number of columns of cells, not vertices.

Output

Return the number of corner-to-corner routes.

Ограничения

0 ≤ rows, columns ≤ 30 and rows + columns ≤ 60. Zero in one dimension gives a straight route. A 0 × 0 grid has one empty route. Counts fit signed 64-bit.

Примеры

Ввод:
rows = 2
columns = 3

Ожидаемый вывод:
expected = 10

Ввод:
long long rows = 2;
long long columns = 3;

Ожидаемый вывод:
long long expected = 10;

Ввод:
long rows = 2;
long columns = 3;

Ожидаемый вывод:
long expected = 10;

Ввод:
rows = 0
columns = 4

Ожидаемый вывод:
expected = 1

Ввод:
long long rows = 0;
long long columns = 4;

Ожидаемый вывод:
long long expected = 1;

Ввод:
long rows = 0;
long columns = 4;

Ожидаемый вывод:
long expected = 1;

Ссылки

Это задание адаптировано для CodeUtility. Следуйте условию и ограничениям выше.

• #15 Lattice Paths