POJ_2407_relatives

#include <iostream>
#include <cmath>
#include <cstring>
#include <windows.h>
using namespace std;

bool isPrime[100000];

void primeRadiation( const int& size ){

memset( isPrime, true, sizeof( isPrime ) );
isPrime[1] = false;

for( int i = 4; i <= size; i += 2 )
isPrime[i] = false;

for( int i = 3; i <= sqrt( size ); ++i ){
if( isPrime[i] ){
const int step = 2 * i;
for( int j = i * i; j <= size; j += step )
isPrime[j] = false;
}
}
}

int main(){

int num;

while( cin >> num ){

if( num == 0 )
break;

double res = num;
int temp = num;
primeRadiation( num );

for( int i = 2; i <= num; ++i ){
if( isPrime[i] && temp % i == 0 ){

res *= ( 1 - 1.0 / i );

while( temp % i == 0 )
temp /= i;

}
}

cout << res << endl;
}

return 0;

}