#include <bits/stdc++.h>
using namespace std;

struct iofast_t {
    iofast_t() {
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
    }
} iofast;

struct uns_t {} uns;
template <typename Element, typename Head, typename ...Args>
auto vec(Element init, Head arg, Args ...args) {
    if constexpr (sizeof...(Args) == 0) return std::vector(arg, init);
    else return std::vector(arg, vec(init, args...));
}
template <typename Element, typename Head, typename ...Args>
auto vec(uns_t, Head arg, Args ...args) {
    return vec(Element(), arg, args...);
}

template <typename T, typename Compare = less<T>>
T &chmin(T &l, T r, Compare &&f = less<T>()) { return l = min(l, r, f); }
template <typename T, typename Compare = less<T>>
T &chmax(T &l, T r, Compare &&f = less<T>()) { return l = max(l, r, f); }

int main() {
    int l, r; cin >> l >> r;

    const int lim = 2 * r;

    auto is_prime = vec<bool>(true, lim + 1);

    is_prime[0] = is_prime[1] = false;
    for (int i = 2; i <= lim / i; ++i) {
        if (!is_prime[i]) {
            continue;
        }
        for (int j = i * i; j <= lim; j += i) {
            is_prime[j] = false;
        }
    }

    auto check = [&](int64_t a, int64_t b, int64_t p) {
        return l <= a && a <= b && b <= r && is_prime[p];
    };

    int ans = 0;
    for (int i = max(2, l); i <= r; ++i) {
        ans += check(2 - i, i, i + i - 1);
        ans += check(i - 1, i, i + i - 1);
        ans += check(1 - i, i, (i + i - 1) / 2) && (i + i - 1) % 2 == 0;
        ans += check(i, i, (i + i) / 2);
    }

    cout << ans << endl;
}