Contest1343/C.cpp

#include<bits/stdc++.h>
 
using namespace std;
using LL=long long;
const int MAXN = 1e6 + 10;
vector<int> primes = {2, 3};
 
void init() {
    for (int k = primes.back()+2; k <= MAXN; k += 2) {
        bool flag = true;
        for (auto p:primes) {
            if (p * p > k)break;
            if (k % p == 0) {
                flag = false;
                break;
            }
        }
        if (flag)primes.push_back(k);
    }
}
 
int main() {
    init();
    for (int n; cin >> n;) {
        int tot = 0;
        for (size_t i = 0; i < primes.size() - 1; i++) {
            if (primes[i + 1] > n)break;
            if (primes[i + 1] - primes[i] <= 2) {
                //cout << primes[i] << " " << primes[i + 1] << endl;
                ++tot;
            }
        }
        cout << tot << endl;
    }
    return 0;
}