What is the next letter FMSSTT?

The sequence "FMSSTT" can be analyzed by looking at the letters in pairs: FM, SS, TT. Each pair consists of the first letter of the next two letters in the English alphabet: F (6), M (13), S (19), T (20). Following this pattern, the next pair should continue with the next letter after T, which is U, so the next letter is U.