Omitting voids and points of punctuation please make a program that will take a text and his each letter will be replaced by his precedent in the alphabet. The program will process a 26 X 26 table which the cells of are considered that they have letters for pointers and no numbers. The price of cell (x,y) represents how much times presents itself the pair of XY in the text.

For example the text will be NOW IS THE TIME FOR MEN TO AID THEIR COUNTRY. it will change to: MNVHRSGDSHLDENQLDMSNZHCSGDHQBNTMSQX
the A m,n=1 and A n,v=1 ... and A s,g=2. Where 1,.2 is their frequency in our text. The last letter of text has as his next the first.

Please help me with thisone, it really means a lot for me.
I would expect soon from you
thank you very much!