{"trustable":true,"prependHtml":"\u003cscript\u003e window.katexOptions \u003d { disable: true }; \u003c/script\u003e\n\u003cscript type\u003d\"text/x-mathjax-config\"\u003e\n MathJax.Hub.Config({\n tex2jax: {\n inlineMath: [[\u0027$$$\u0027,\u0027$$$\u0027], [\u0027$\u0027,\u0027$\u0027]],\n displayMath: [[\u0027$$$$$$\u0027,\u0027$$$$$$\u0027], [\u0027$$\u0027,\u0027$$\u0027]]\n }\n });\n\u003c/script\u003e\n\u003cscript async src\u003d\"https://mathjax.codeforces.org/MathJax.js?config\u003dTeX-AMS-MML_HTMLorMML\" type\u003d\"text/javascript\"\u003e\u003c/script\u003e","sections":[{"title":"","value":{"format":"HTML","content":"\u003cdiv class\u003d\"panel_content\"\u003eIf we define dn as: dn \u003d pn+1-pn, where pi is the i-th prime. It is easy to see that d1 \u003d 1 and dn\u003deven for n\u0026gt;1. Twin Prime Conjecture states that \"There are infinite consecutive primes differing by 2\".\u003cbr\u003eNow given any positive integer N (\u0026lt; 10^5), you are supposed to count the number of twin primes which are no greater than N.\u003c/div\u003e"}},{"title":"Input","value":{"format":"HTML","content":"Your program must read test cases from standard input.\u003cbr\u003eThe input file consists of several test cases. Each case occupies a line which contains one integer N. The input is finished by a negative N."}},{"title":"Output","value":{"format":"HTML","content":"For each test case, your program must output to standard output. Print in one line the number of twin primes which are no greater than N."}},{"title":"Sample","value":{"format":"HTML","content":"\u003ctable class\u003d\u0027vjudge_sample\u0027\u003e\n\u003cthead\u003e\n \u003ctr\u003e\n \u003cth\u003eInput\u003c/th\u003e\n \u003cth\u003eOutput\u003c/th\u003e\n \u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\u003cpre\u003e1\r\n5\r\n20\r\n-2\u003c/pre\u003e\u003c/td\u003e\n \u003ctd\u003e\u003cpre\u003e0\r\n1\r\n4\u003c/pre\u003e\u003c/td\u003e\n \u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n"}}]}