#include <iostream> #include <vector> #include <set> using namespace std; vector<long long> p; set<long long> s; bool used[500010]; void prime(int n){ int i,j; for(i=2;i<=n;i++){ used[i] = true; } for(i=2;i<=n;i++){ if(!used[i]) continue; for(j=2*i;j<=n;j+=i){ used[j] = false; } } } int main(){ int i,n,cnt = 1; cin >> n; if(n==1){ cout << 0 << endl; return 0; } prime(n); for(i=3;i<=n;i++){ if(used[i]){ p.push_back(i); s.insert(i); } } for(i=0;i<p.size();i++){ if(p[i]*p[i] - 2>n) break; if(s.count(p[i]*p[i] - 2)) cnt += 2; } cout << cnt << endl; }