I solving the problems at Project Euler

I'm at problem 2

and I have made this program so far

but I dont know why it takes too much time to give me the answer

even if the fibonacci number is 4

the code is this

any hint on how to improve would be really appreciatedCode:`#include <iostream>`

using namespace std;

int main ()

{

typedef unsigned long ul;

ul n1 = 0;

ul n2 = 1;

ul n3;

ul index1 = 0;

long double sum = 0;

while (index1 < 4) {

n3 = n1 + n2;

if ( n3 % 2 == 0 ) { sum += n3; }

n1 = n2;

n2 = n3;

}

cout << sum;

return EXIT_SUCCESS;

}

thanks