C program to check prime number

 

Prime Number formula: For a number p>3; if  p² - 1 is divisible by 24 then it is prime.

 

#include<iostream>

using namespace std;

int main()

{

int n;

cin>>n;

if(n>=1){

if(n==2 || n==3){

cout<<n<<" is prime";

}

else if(((n*n)-1)%24==0){

cout<<n<<" is prime";

}

else{

cout<<n<<" is not prime";

}

}

else{

cout<<"Enter a valid number";

}

return 0;

}