def fib(n):
    if n == 0:
        return 0
    else:
        a, b = 0, 1
        for i in range(2, n + 1):
            a, b = b, a + b
        return b

end = int(input())

if end < 0:
    print("меньше 0")
else:
    count = 0
    sum = 0
    while fib(count) <= end:
        sum = sum + fib(count);

        count += 1
print(sum)