昨天的Atcoder Beginner Contest 的F题实在不会，题解都看不懂，能不能大致给一个思路？

How many strings can be obtained by applying the following operation on a string $S$ exactly $K$ times: "choose one lowercase English letter and insert it somewhere"?

The answer can be enormous, so print it modulo $(10^9+7)$ .

• $K$ is an integer between $1$ and $10^6$ (inclusive).
• $S$ is a string of length between $1$ and $10^6$ (inclusive) consisting of lowercase English letters.