To print all prime numbers between two given numbers m and n

The need:
    This program was asked me by a reader of this blog. This program takes 2 numbers from the user and prints all the prime numbers between them (inclusive both the numbers).
The code:
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#include<stdio.h>
#include<math.h>
int isPrime(int );
main()
{
    int number1,i,number2;
    printf("enter a natural number.\n");
    scanf("%d",&number1);
    printf("enter second natural number.\n");
    scanf("%d",&number2);
    if(number1<1||number2<1)
    {
        printf("one of the numbers entered was not a natural number.\n");
        exit(0);
    }
    printf("the prime numbers between %d and %d are =>\n\n",number1,number2);
    for(i=number1;i<=number2;i++)
    {
        if(isPrime(i))
        {
            printf("%d\t",i);
        }
    }
    printf("\n\n");
}

int isPrime(int x)            //this function returns one if the
{                            //input number x is prime else returns 0
    int i,flag=1;
    if(x==1)   //people say 1 is not a prime number
     return 0;
    else if(x==2)  //people say 2 is a prime number
     return flag;
    else    //just following the definition for the number greater than 2
    for(i=2;i<=sqrt(x);i++)
    {
        if(x%i==0)
        {
            flag=0;
            break;
        }
    }
    return flag;
}

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Remarks:

  If you are using linux/unix to compile this program, add '-lm'(without quotes) after the compile command. Ex. : if the file name is file.c, give the command:

"cc file.c -lm"(without quotes)
instead of only "cc file.c"