how to check wether the given number is prime or not

#include<stdio.h>
#include<math.h>
main()
{
int i,flag,n;
printf("enter your number:");
scanf("%d",&n);
flag=1;
for(i=2;i<=sqrt(n);i++)
{
if(n%i==0)
{
flag=0;
break;
}
}
if(flag==0)
printf("\n %d is not a prime number",n);


else
printf("\n %d is a prime number",n);
}