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Length of a Sumac Sequence

Heavily based on [this closed challenge from SE.](https://codegolf.stackexchange.com/q/148911/80214)
# Description
A Sumac sequence starts with two non-zero integers \$t_1\$ and \$t_2.\$
The next term, \$t_3 = t_1 - t_2\$
More generally, \$t_n = t_{n-2} - t_{n-1}\$
The sequence ends when \$t_n ≤ 0\$. All values in the sequence must be positive.
# Challenge
Given two integers \$t_1\$ and \$t_2\$, compute the Sumac sequence, and output it's length.
If there is a negative number in the input, remove everything after it, and compute the length.
You may take the input in any way (Array, two numbers, etc.)
# Test Cases
(Sequence is included for clarification)
```
[t1,t2] Sequence n
------------------------------
[120,71] [120,71,49,22,27] 5
[101,42] [101,42,59] 3
[500,499] [500,499,1,498] 4
[387,1] [387,1,386] 3
[3,-128] [3] 1
[-2,3] [] 0
[3,2] [3,2,1,1] 4
```
# Scoring
~~This is [tag: code-golf]. Shortest answer in each language wins.~~

Heavily based on [this closed challenge from SE.](https://codegolf.stackexchange.com/q/148911/80214)
# Description
A Sumac sequence starts with two non-zero integers \$t_1\$ and \$t_2.\$
The next term, \$t_3 = t_1 - t_2\$
More generally, \$t_n = t_{n-2} - t_{n-1}\$
The sequence ends when \$t_n ≤ 0\$. All values in the sequence must be positive.
# Challenge
Given two integers \$t_1\$ and \$t_2\$, compute the Sumac sequence, and output it's length.
If there is a negative number in the input, remove everything after it, and compute the length.
You may take the input in any way (Array, two numbers, etc.)
# Test Cases
(Sequence is included for clarification)
```
[t1,t2] Sequence n
------------------------------
[120,71] [120,71,49,22,27] 5
[101,42] [101,42,59] 3
[500,499] [500,499,1,498] 4
[387,1] [387,1,386] 3
[3,-128] [3] 1
[-2,3] [] 0
[3,2] [3,2,1,1] 4
```
# Scoring
This is <a class="badge is-tag">code-golf</a>. Shortest answer in each language wins.

code-golf math sequence