PROGRAMMING:Brother Chenyang's distress

Chen Yang's brother and his girlfriend are shopping in the street. When Chen Yang's brother's girlfriend sees twins playing in the street, she says with emotion: how can there be no twins in the number... Brother Chenyang, who had seen a lot of things, immediately replied, "yes, in 1849, alfon? virtue? Poliniak proposed a general conjecture: for all natural numbers K, there are infinitely many prime pairs (P, P + 2K). The case of k = 1 is the twin prime conjecture. Chenyang brother's girlfriend was very happy and said: I'll give you a whole number, and you can help me calculate how many pairs of twin prime numbers are less than or equal to this number ~ ~ this is troubling our Chenyang brother. Of course, it's not difficult to calculate this, but the trouble is that our Chenyang brother has urgent things to do and can't help his girlfriend, But he has a girlfriend who doesn't want to break his promise, so he has to come to you for help. Smart acmer, you will certainly help brother Chenyang, right?
###Input format:
There are several groups of test data, which are processed to the end of the file. Enter a positive integer n (n < 1000).
###Output format:
For each group of inputs, the output is the logarithm of twin primes in the range of n (less than or equal to n). Each output occupies one line.
###Input example:
Here is a set of inputs. For example:
```in
one
five
twenty
```
###Output example:
The corresponding output is given here. For example:
```out
0
one
four
```







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