#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<math.h>
#pragma warning(disable:4996)

int ss(long long int k)
{
    k = k > 2 ? k : 2;
    for (long long int i = 2; i <= (sqrt(k) + 1); i++)
    {
        if (k%i==0)
        {
            return 0;
        }
    }
    return 1;
}


int main(void)
{
    long long int m, n, a = 0, b, c;
    scanf("%lld %lld", &m, &n);
    b = m < n ? m : n;
    c = m > n ? m : n;
    for (long long int i = b; i <= c; i++)
    {
        if (ss(i))
        {
            a++;
        }
    }
    printf("%lld", a);
    return 0;
}