Twin Primes in a Range

/**
* This program inputs 2 numbers and prints all the twin prime numbers within that range.
*/
import java.util.Scanner;
public class TwinPrimes
{
    public static boolean isPrime(int num) //Function to check whether a number is a prime number or not.
    {
        if(num == 2) //Since 2 is the first prime number.
        {
            return true;
        }
        else
        {
        int i, flag=1;
        for(i=2;i<num;i++)
        {
            if(num%i==0)
            {
                flag=0;
                break;
            }
        }
        if(flag == 1)
        {
            return true;
        }
        else
        {
            return false;
        }
        }
    }
    public static void twinPrimes(int start, int end) //Function to find twin primes in a range.
    {
        int i;
        boolean c1,c2;
        System.out.println("Twin Primes between "+start+" and "+end+" are:");
        for(i=start+1;i<end-1;i++)
        {
            c1=isPrime(i); //Checking whether a number is prime or not.
            if(c1==true)
            {
                c2=isPrime(i+2); //If the number above is a prime, we will check whether (number + 2) is a prime as well or not.
                if(c2==true) //A pair of twin primes is found!
                {
                    System.out.println("( "+i+","+(i+2)+" )");
                }
            }
        }
    }
    public static void main(String args[])
    {
        Scanner sc=new Scanner(System.in);
        System.out.println("Please enter the lower limit: ");
        int start=sc.nextInt();
        System.out.println("Please enter the upper limit: ");
        int end=sc.nextInt();
        if(start > end) //Checking whether the user is an idiot or not.
        {
            System.out.println("Incorrect Input. The lower limit must be lesser than the upper limit.");
        }
        else
        {
            twinPrimes(start,end);
        }
        sc.close();
    }
}