878\. Nth Magical Number

Hard

A positive integer is _magical_ if it is divisible by either `a` or `b`.

Given the three integers `n`, `a`, and `b`, return the <code>n<sup>th</sup></code> magical number. Since the answer may be very large, **return it modulo** <code>10<sup>9</sup> + 7</code>.

**Example 1:**

**Input:** n = 1, a = 2, b = 3

**Output:** 2

**Example 2:**

**Input:** n = 4, a = 2, b = 3

**Output:** 6

**Constraints:**

*   <code>1 <= n <= 10<sup>9</sup></code>
*   <code>2 <= a, b <= 4 * 10<sup>4</sup></code>