Friend or Girlfriend

Shlok and Sachin are good friends. Shlok wanted to test Sachin, so he wrote down a string $S$ with length $N$ and one character $X$. He wants Sachin to find the number of different substrings of $S$ which contain the character $X$ at least once. Sachin is busy with his girlfriend, so he needs you to find the answer. Two substrings of $S$ are considered different if their positions in $S$ are diff