1 #include<cstdio> 2 #include<cmath> 3 #include<cstring> 4 #include<algorithm> 5 using namespace std; 6 typedef long long ll; 7 const int N = 1000001; 8 int prime[N+1]; 9 void getPrime(){ 10 memset(prime, 0, sizeof(prime)); 11 for(int i = 2;i <= N; i++){ 12 if(!prime[i]) 13 prime[ ++prime[0] ] = i; 14 for(int j = 1; j <= prime[0] && prime[j] <= N / i; j++){ 15 prime[ prime[j] * i ] = 1; 16 if(i % prime[j] == 0) break; 17 } 18 } 19 } 20 int main(){ 21 getPrime(); 22 int i, n, f; 23 scanf("%d", &n); 24 f = upper_bound(prime+1, prime + 1 + prime[0], n-1) - prime; 25 f = upper_bound(prime+1, prime + 1 + prime[0], f-1) - prime; 26 printf("%d ", prime[prime[f]]); 27 return 0; 28 }