Binary Strings with at most K Consecutive One in Rust

Given two non-negative integers N and K, return the number of binary strings of length N with at most K consecutive ones

Example:

Input: N = 4, K = 2
Output: 13
Explanation: [
    "0000",
    "0001",
    "0010",
    "0011",
    "0100",
    "0101",
    "0110",
    "1000",
    "1001",
    "1010",
    "1011",
    "1100",
    "1101",
]
"0111", "1110" and "1111" are not valid because they have more than 2 consecutive ones