#include <stdio.h>

int main(void)
{
    int a, b;
    int i, j;
    int count = 0;
    int prime;

    scanf("%d %d", &a, &b);

    for (i = a; i <= b; i++)
    {
        if (i >= 2)
        {
            prime = 1;

            for (j = 2; j < i; j++)
            {
                if (i % j == 0)
                {
                    prime = 0;
                    break;
                }
            }

            if (prime == 1)
            {
                count++;
            }
        }
    }

    printf("%d\n", count);

    return 0;
}