10200.cpp

#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;

const int LIM = 10001;

int Cal(int N){
	return ( N * N + N + 41 );
}

bool isPrime(int N){
	for (int i=2; i*i<=N; i++)
		if( N%i == 0 )
			return false;
	return true;
}

int main(){
	int prime[LIM+1];
	int i;
	memset(prime, 0, sizeof prime);
	prime[0] = 1;
	for (i=1; i<LIM; i++){
		bool stat = isPrime( Cal(i) );
		prime[i] = prime[i-1];
		if( stat )
			prime[i]++;
	}
	int A, B;
	while( scanf("%d%d", &A, &B) != EOF ){
		int val = 0;
		val = prime[B];
		if( A > 0 )
			val -= prime[A-1];
		double per = (double)val / (double)(B - A + 1);
		per *= 10000;
		per = floor( per + 0.5 );
		per /= 100;
		printf("%.2lf\n", per);
	}
	return 0;
}