#include <iostream>
#include <vector>
using namespace std;

int main() {
    int n;
    cin >> n;

    vector<bool> is_prime(n + 1, true);
    is_prime[0] = is_prime[1] = false;

    // 埃氏筛法筛素数
    for (int i = 2; i * i <= n; ++i) {
        if (is_prime[i]) {
            for (int j = i * i; j <= n; j += i) {
                is_prime[j] = false;
            }
        }
    }

    int max_len = 0;
    int current_len = 0;

    for (int i = 1; i <= n; ++i) {
        if (!is_prime[i]) {
            current_len++;
            max_len = max(max_len, current_len);
        } else {
            current_len = 0;
        }
    }

    cout << max_len << endl;

    return 0;
}