#include<bits/stdc++.h>
#define MAX 100010050

using namespace std;

bool check[MAX];

int cum_sum[10005];

void sieve()
{
    int i,j,k=sqrt(MAX);

    for(i=41;i<k;i+=2)
    {
        for(j=i*i;j<MAX;j+=i+i)
        {
            check[j] = true;
        }
    }

    cum_sum[0] = 1;

    cum_sum[1] = 2;

    for(i=2;i<=10000;i++)
    {
        if(!check[(i*i)+i+41])
        {
            cum_sum[i] = 1;
        }

        cum_sum[i]+=cum_sum[i-1];
    }
}

int main()
{
    sieve();

    int a,b;

    double res,store,tot;

    while(scanf("%d%d",&a,&b)!=EOF)
    {
        tot = b-a+1;

        store = cum_sum[b]-cum_sum[a-1];

        res = (store/tot)*100.0;

        printf("%.2lf\n",res+1e-7);
    }

    return 0;
}