1. It is required to decipher what numbers are encrypted, if the same digits are replaced by the same letters:

$$
\text { KIS }+ \text { KSI = ISK. }
$$
1. 

$$
\text { KIS+ KSI = ISK. }
$$

From the fact that two three-digit numbers are added and the result is again a three-digit number, we can conclude that I $\neq 0$ and $K<5$, meaning K can take the values $1,2,3$ or 4. We get the following options:

$$
\begin{aligned}
& 1 I S+1 S I=I S 1 ; \\
& 2 I S+2 S I=I S 2 ; \\
& 3 I S+3 S I=I S 3 ; \\
& 4 I S+4 S I=I S 4 .
\end{aligned}
$$

Considering each option sequentially, we will find the solution. We will show how to conduct the reasoning on one of them:

$$
\begin{array}{rrr}
2 & I & \text { S } \\
2 & \text { S } I \\
\hline
\end{array}
$$

The hundreds digit in the sum can be 4 or 5. Let's consider each case. Suppose $I=4$, we get

$$
\begin{array}{rrr}
2 & 4 & \mathrm{S} \\
+ & \mathrm{S} & 4 \\
\hline 4 & \mathrm{S} & 2
\end{array}
$$

From the obtained example, it is easy to determine $\mathrm{S}=8$, but substituting $\mathrm{S}=8$ into the tens place results in a contradiction ( $4+8 \neq 8-1$ ).

Suppose $I=5$, we get

![](https://cdn.mathpix.com/cropped/2024_05_21_e53d4ff4f2278a1c0d12g-166.jpg?height=205&width=276&top_left_y=1485&top_left_x=896)

From the obtained example, it is easy to determine $\mathrm{S}=7$, but substituting into the tens place results in a contradiction ( $5+7 \neq 7-1$ ).

Considering the remaining cases similarly, we find the solution: $495+459=954$.

Answer: $K=4, I=9, S=5$.