Rabbits

#include <bits/stdc++.h>

using namespace std;

/*
Far far away in the future, humanity has sent 1 frozen newborn rabbit pair to populate an alien planet. Each pair of rabbits reaches maturity at the age of 1 month and they immediately mate. One month after mating the pair reproduces one pair of rabbits, after which the older pair mates again immediately. The rabbits never die and continue breeding forever.
How many months will the rabbit population be in an interval between a and b?
Input:
The first line contains 2 integers a,\,b: the interval boundaries.
Output:
Print a single integer d: the duration (in months) when the number of rabbit pairs is in the interval [a,\,b] inclusive.

Constraints:
1 \leq a \leq b \leq 10^{100}*/

long long fibonacci (long long i) {
  if (i == 0)
    return 0;
  if (i == 1)
    return 1;
  return fibonacci (i - 1) + fibonacci (i - 2);
}

int main()
{
    // ios_base::sync_with_stdio(0);
    // cin.tie(0);

    int a, b;
    cin >> a >> b;

    int x = 0;
    while (fibonacci(x) < a)
        ++x;

    long long count = 0;

    while (fibonacci(x) <= b) {
        ++x;
        ++count;
    }

    cout << count << endl;
}

Test 1

Verdict: TIME LIMIT EXCEEDED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: TIME LIMIT EXCEEDED

Test 4

Verdict: TIME LIMIT EXCEEDED

Test 5

Verdict: TIME LIMIT EXCEEDED

Test 6

Verdict: TIME LIMIT EXCEEDED

Test 7

Verdict: TIME LIMIT EXCEEDED

Test 8

Verdict: TIME LIMIT EXCEEDED

Test 9

Verdict: TIME LIMIT EXCEEDED

Test 10

Verdict: TIME LIMIT EXCEEDED

Test 11

Verdict: TIME LIMIT EXCEEDED

Test 12

Verdict: TIME LIMIT EXCEEDED

Test 13

Verdict: TIME LIMIT EXCEEDED

Test 14

Verdict: TIME LIMIT EXCEEDED

Test 15

Verdict: TIME LIMIT EXCEEDED

Test 16

Verdict: TIME LIMIT EXCEEDED

Test 17

Verdict: TIME LIMIT EXCEEDED

Test 18

Verdict: TIME LIMIT EXCEEDED

Test 19

Verdict: TIME LIMIT EXCEEDED

Test 20

Verdict: TIME LIMIT EXCEEDED

Test 21

Verdict: TIME LIMIT EXCEEDED

Test 22

Verdict: TIME LIMIT EXCEEDED

Test 23

Verdict: TIME LIMIT EXCEEDED