PRIMETIME.c

#include <stdio.h>
#include <math.h>

long isprime(long n);
long n2n41(long n);
int primepct[10001];

int main(int argc, char **argv)
{
	long a, b, i, ip;
	int notprime, prime;
	for(i = 0; i < 10001; i++)
	{
			primepct[i] = 0;
	}
	while (scanf("%ld %ld", &a, &b) != EOF)
	{
		for(i = a; i <= b; i++)
		{
			if(primepct[i-1]==0)
			{				
				if(isprime(n2n41(i))==0)
				{
					prime++;
					primepct[i-1] = 1;
				}
				else
				{
					notprime++;
					primepct[i-1] = -1;
				}
			}
			else
			{
				if(primepct[i-1]==1)
				{
					prime++;
				}
				else
				{
					notprime++;
				}
			}
		}
		printf("%.2f\n", floor(((double)prime/(prime+notprime))*10000+.5)/100);
		notprime = 0;
		prime=0;
	}
	return 0;
}

long n2n41(long n)
{
	return n*n+n+41;
}

long isprime(long n)
{
	long i = 3;
	long sqn = 0;
	
	while (n%2==0)
	{
		n /= 2;
		return 1;
	}
	sqn = (sqrt(n))+1;
	while (i <= sqn)
	{
		if ((n%i)==0)
		{
			n /= i;
			return 1;
		}
		else
		{
			i += 2;			
		}
	}
	return 0;
}