Zawad and Index Sum

Zawad is a rare competitive programmer — he takes his studies seriously. One day, while studying in a library, he found a pattern $P$ in a century old book. The pattern is a string of lowercase English letters, with length $M$. Since Zawad is a curious boy, he started experimenting with the pattern. In one experiment, Zawad considers all $26^N$ strings with length $N$ that contain only lowercase