Sum of prime numbers below 2 million - Optimization?

[isxj]

Hi, this is my attempt to calculate the sum of all prime numbers bellow 2000000.
Even though it's kinda fast (took about 6.5 seconds in Code::Blocks) I was wondering how could it be optimized.

Also, what's your opinion on my attempt?
Thanks in advance!

#include <iostream>
#include <math.h>
using namespace std;

bool isPrime(int);
long long primeSum=1;

int main()
{
    for(int n=1; n<2000000; n+=2){
	if (isPrime(n)==true) primeSum+=n;  
    }

    cout << primeSum;
}

bool isPrime(int num)
{
    for(int d=1; d<=sqrt(num); d+=2){
        if (num%d==0 and d!=1){
            return false;
        }
    }
    return true;
}

[Peter87]

Have you turned on compiler optimizations?

For me your program runs in 1.1 seconds without compiler optimizations (-O0) and in 0.6 seconds with optimizations turned on (-O2).

[isxj]

Well, I thought that it was already optimized xD.
But I changed the settings and it took 0.37 seconds (-O2)!

Thanks!