Pascal's Triangle I

Given two integers r and c, return the value at the rth row and cth column (1-indexed) in Pascal's Triangle.

In Pascal's Triangle:

• The first row contains a single element, 1.

• Each row has one more element than the previous row.

• Every row starts and ends with 1.

• For all interior elements (not at the ends), the value at position (r, c) is the sum of the two elements directly above it from the previous row:

  Pascal[r][c] = Pascal[r−1][c−1] + Pascal[r−1][c]

  where indexing is 1-based.

Example 1

• Input : r = 4, c = 2
• Output : 3
• Explanation : Row 4 of Pascal's Triangle is 1 3 3 1 (built as 1 → 1 1 → 1 2 1 → 1 3 3 1), so the value at row 4, column 2 is 3.

Example 2

• Input : r = 5, c = 3
• Output : 6
• Explanation : Row 5 of Pascal's Triangle is 1 4 6 4 1 (built as 1 → 1 1 → 1 2 1 → 1 3 3 1 → 1 4 6 4 1), so the value at row 5, column 3 is 6.

Constraints

• 1 <= r, c <= 30
• c <= r
• All values will fit inside a 32-bit integer.