{"trustable":false,"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":"MD","content":"丁丁妹从小就表现出对数学的强烈兴趣。这天,她刚学会质数的概念:如果一个数的因数只有1和它自己,那么它是一个质数。善于思考的她提出了这么一个问题:存在多少对不超过N的两个质数,他们的差为2呢?\n"}},{"title":"Input","value":{"format":"MD","content":"多组数据输入。每组数据占一行,每行一个整数N(N\u003c10^5),意义如上。输入数据以一个负数作为结尾。"}},{"title":"Output","value":{"format":"MD","content":"对每组数据,输出一行,每行一个整数,表示不超过N且相差为2的质数对数。"}},{"title":"Sample Input","value":{"format":"MD","content":"1\n6\n-999"}},{"title":"Sample Output","value":{"format":"MD","content":"0\n1"}}]}