long sum = 0;
        long x1 = 1, x2 = 2;
        while (x2 <= n) {
            if (x2 % 2 == 0)
                sum += x2; 
            long next = x1 + x2; 
            x1 = x2;         
            x2 = next;
        }
        printf("%ld\n",sum);