Problem
A digit string is good if the digits (0-indexed) at even indices are even and the digits at odd indices are prime (2
, 3
, 5
, or 7
).
- For example,
"2582"
is good because the digits (2
and8
) at even positions are even and the digits (5
and2
) at odd positions are prime. However,"3245"
is not good because3
is at an even index but is not even.
Given an integer n
, return the total number of good digit strings of length n
. Since the answer may be large, return it modulo 109 + 7
.
A digit string is a string consisting of digits 0
through 9
that may contain leading zeros.
Example 1:
Input: n = 1 Output: 5 Explanation: The good numbers of length 1 are "0", "2", "4", "6", "8".
Example 2:
Input: n = 4 Output: 400
Example 3:
Input: n = 50 Output: 564908303
Constraints:
1 <= n <= 1015