Problem:
Score: 300 points

Problem Statement

Find the minimum prime number greater than or equal to X.

Notes

A prime number is an integer greater than 1 that cannot be evenly divided by any positive integer except 1 and itself.

For example, 2, 3, and 5 are prime numbers, while 4 and 6 are not.

Constraints

2 \le X \le 10^5

All values in input are integers.

Input

Input is given from Standard Input in the following format:

X

Output

Print the minimum prime number greater than or equal to X.

Sample Input 1

20

Sample Output 1

23

The minimum prime number greater than or equal to 20 is 23.

Sample Input 2

2

Sample Output 2

2

X itself can be a prime number.

Sample Input 3

99992

Sample Output 3

100003

Solution in C++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
#include <algorithm>
#include <vector>
#include <cstdio>
#include <cmath>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <cstring>
#include <cstdlib>
using namespace std;

#define REP(i,n) for(int i = 0; i < (int)(n); i++)
typedef long long ll;
typedef pair<int, int> pint;
template <class T> inline void chmax(T &a, const T b) { a = max(a, b); }
template <class T> inline void chmin(T &a, const T b) { a = min(a, b); }
template <class T> void pp(T v, int n) { REP(i,n) cout<<v[i]<< ' '; cout << endl; }

void inline TorF(bool tf, const string &trueMessage, const string &falseMessage) { 
  cout << (tf ? trueMessage : falseMessage) << endl;
}
void inline YESNO(bool tf) { TorF(tf, "YES", "NO"); }
void inline YesNo(bool tf) { TorF(tf, "Yes", "No"); }

bool isprime(int n) {
  if(n <= 1) return false;
  for(int i = 2; i * i <= n; i++) {
    if(n % i == 0) return false;
  }
  return true;

}

int main() {
  int X ; cin>>X;
  for( ; ; X++) {
    if(isprime(X)) {
      cout << X << endl;
      break;
    }
  }
  return 0;
}