Find out how many such pair of letters are there in the word RUST that have as many letters between them in the word as there are between them in the English alphabet: RUST

Let us assign alphabetical positions to each letter in the word $\text{RUST}$:
* $\text{R}=18$
* $\text{U}=21$
* $\text{S}=19$
* $\text{T}=20$

Now, let's scan for valid letter pairs in both forward and backward directions:

1. Forward Direction:
* From $\text{R}(18)$: $\text{RU}$ (diff $3\ne 1$), $\text{RS}$ (diff $1\ne 2$), $\text{RT}$ (diff $2\ne 3$). No pair.
* From $\text{U}(21)$: $\text{US}$ (no), $\text{UT}$ (no).
* From $\text{S}(19)$: $\text{ST}$ ($19\to 20$). In the English alphabet, $\text{S}$ and $\text{T}$ are consecutive with zero letters between them, which matches their positions in the word $\text{RUST}$. This is $1$ valid pair ($\text{ST}$).

2. Backward Direction:
* From $\text{T}(20)$: $\text{TS}$ (no), $\text{TU}$ (no), $\text{TR}$ (no).
* From $\text{S}(19)$: $\text{SU}$ (diff $2\ne 1$), $\text{SR}$ (no).
* From $\text{U}(21)$: $\text{UR}$ (no).

Thus, there is only one such pair ($\text{ST}$).