Project Euler 2# - strange output - c

Question

I've tried to solve problem 2 on Project Euler in C. This is the first possible solution that came to my mind, and, in fact, it gives as an output the right answer. The problem is that each time I run my program it gives me a different output, which is either "2" or "4613732" that is the right answer. Sorry for my poor english, can you help me find out what's wrong?
#include <stdio.h>
int main(){
int n, n1 = 1, n2 = 2, sum = 2;
while(n<4000000){
n = n1 + n2; /*calculate the next number of the series*/
n1 = n2;
n2 = n;
if(n%2 == 0){
sum = sum + n; /*if the number it's even add it to the main sum*/
}
}
printf("The sum is %d\n", sum);
}

Answer 1

You didn't initialize n; when you get the right answer, it means you got lucky.

Answer 2

#include <conio.h>
#include <iostream>
using namespace std;
int evenFibSum(int i=1,int j=2)
{
const int max = 3999999;
int eventsum = 2;
int sum = 0;
while (sum < max)
{
sum = i + j;
i = j;
j = sum;
if (sum % 2 == 0)
eventsum +=sum;
}
return eventsum;
}
For more efficient solution apply following logic
Fibbonaci Series => 1 2 3 5 8 13 21 34 55 89 144
Index => 0 1 2 3 4 5 6 7 8 9 10
To get even fibbonanci addition I have to add following index values[1 4 7 10]
Here I am sensing some pattern
[1 4 7 10] => I need advance index by 3
so how to advance index by 3
// k = i+j = > 3 13 55
// i = k+j => 5 21 89
// j = k+i => 8 34 144
int evenFibSumx(int i=1,int j=2)
{
const int max = 3999999;
int eventsum = 2;
int k= 0;
while (1)
{
k = i + j;
i = k + j;
j = k + i;
if(i >= max)
break;
if (j%2 == 0)
eventsum +=j;
}
return eventsum;
}
int main()
{
std::cout << evenFibSum();
std::cout << evenFibSumx();
}