#include <stdio.h>
int main() {
int a, b, count = 0;
scanf("%d%d", &a, &b);
if (a > b) {
int temp = a;
a = b;
b = temp;
}
for (int i = a; i <= b; i++) {
if (i == 1)
continue;
else {
int j = 0;
for (int c = 2; c * c < i; c++) {
if (i % c == 0) j++;
}
if (j == 0) count++;
}
}
printf("%d", count);
return 0;
}