A twin-prime pair is a pair of primes that differ by exactly 2, such as (3, 5) or (11, 13). Given an upper bound n, count how many twin-prime pairs (p, p+2) exist where both p and p+2 are at most n.
Input format
A single line with one integer n.
Output format
A single integer: the number of twin-prime pairs whose larger member does not exceed n.
Constraints
- 1 <= n <= 1000000