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AOJ0009.cpp

#include <cmath>
#include <iostream>
#include <iomanip>
#include <stack>
#include <list>
#include <vector>

int main(void) {
    std::list<int> input;
    std::vector<int> pnums;
    int buf;

    // generate prime numbers, using sqrt.
    pnums.push_back(2);
    pnums.push_back(3);
    for(int i = 5; i < 1000000; i+=2) {
	buf = std::sqrt(i);
	bool flag = true;
	std::vector<int>::iterator vitr = pnums.begin();
	for(;*vitr <= buf && vitr != pnums.end(); ++vitr) {
	    if(i % *vitr == 0) {
		flag = false;
		break;
	    }
	}
	if(flag) {
	    pnums.push_back(i);
	}
    }

    // test code.
    /*
    std::vector<int>::iterator vitr = pnums.begin();
    for(;vitr != pnums.end(); vitr++) {
	std::cout << *vitr << std::endl;
    }
    */

    // input
    for(;std::cin >> buf;) {
	input.push_back(buf);
    }

    // search.
    std::list<int>::iterator litr = input.begin();
    for(;litr != input.end(); ++litr) {
	int i;
	bool flag;
	std::vector<int>::iterator vitr = pnums.begin();
	for(i = 0, flag = true;vitr != pnums.end(); ++vitr, ++i) {
	    if(*vitr > *litr) break;
	}
        // output
	std::cout << i << std::endl;
    }

    return 0;
}