project euler #10: optimization with primes

[Michael P.]

Hey, I've started to do some of the problems on Project Euler to practice my programming skills. I'm doing #10 right now, and I think I've got a working solution. It's just that it's way too slow.
Here's the link:
http://projecteuler.net/index.php?section=problems&id=10
The code works fine with sum of primes below 10. It output 17.
But for 2 million, it takes quite a while, and an FAQ on the page said most problems are made with being run within a minute. Mine was gonna quite a bit longer.
Here is my code:

//Import
import std.stdio;
import std.math;

//Main
int main()
{
	int sum = 2;
	long bigNum = 10; //Or 2 million
	for ( int i = 3; i < bigNum; i += 2 )
	{
		if( isPrime( i ) )
		{
			sum += i;
		}
	}
	writefln( sum );
	return 0;
}

//Functions
bool isPrime( long n )
{
	for( int i = 2; i < n; i++ )
	{
		if ( n % i == 0 )
		{
			return false;
		}
	}
	return true;
}

I'm pretty sure the isPrime() function can be done better.