There are `n` rings and each ring is either red, green, or blue. The rings are distributed across ten rods labeled from `0` to `9`.

You are given a string `rings` of length `2n` that describes the `n` rings that are placed onto the rods. Every two characters in `rings` forms a color-position pair that is used to describe each ring where:

- The first character of the `ith` pair denotes the `ith` ring's color (`'R'`, `'G'`, `'B'`).
- The second character of the `ith` pair denotes the rod that the `ith` ring is placed on (`'0'` to `'9'`).

For example, `"R3G2B1"` describes `n == 3` rings: a red ring placed onto the rod labeled 3, a green ring placed onto the rod labeled 2, and a blue ring placed onto the rod labeled 1.

Return the number of rods that have all three colors of rings on them.

Example 1:

    Input: rings = "B0B6G0R6R0R6G9"
    Output: 1
    Explanation:
    - The rod labeled 0 holds 3 rings with all colors: red, green, and blue.
    - The rod labeled 6 holds 3 rings, but it only has red and blue.
    - The rod labeled 9 holds only a green ring.
    Thus, the number of rods with all three colors is 1.

Example 2:

    Input: rings = "B0R0G0R9R0B0G0"
    Output: 1
    Explanation:
    - The rod labeled 0 holds 6 rings with all colors: red, green, and blue.
    - The rod labeled 9 holds only a red ring.
    Thus, the number of rods with all three colors is 1.

Example 3:

    Input: rings = "G4"
    Output: 0
    Explanation:
    Only one ring is given. Thus, no rods have all three colors.

Constraints:

- `rings.length == 2 * n`
- `1 <= n <= 100`
- `rings[i]` where `i` is even is either `'R'`, `'G'`, or `'B'` (0-indexed).
- `rings[i]` where `i` is odd is a digit from `'0'` to `'9'` (0-indexed).