[SOLVED] A Question about RSA

I don't know if the algorithm of RSA can be posted here~~

here's my question

what should i do if i calculate a negative secrect key?

for example:

n = 77; a = 13;

the public key is ( a, ), which is( 13, 60 )

so, 60 = 13x4 + 8

13 = 8x1 + 5

8 = 5x1 + 3

5 = 3x1 + 2

3 = 2x1 + 1

and by euclidean, got 1 = 5x60 - 23x13

and according to the algorithm aX - Y = 1 , where X >= 1 & Ｙ＞＝１, i just got x = -23; y = -5.

then what should i do now?

i found a solution that to make them positive, x' = x+kb; y' =y-ka

but what's the b here? the block size? if it is, then what's the block size of this:

{ encode letters ‘A’ to ‘Z’ as 10 to 35. using one block for each letter and use RSA to encrypt ‘EULER’。so, EULER = 14 30 21 14 27 。}

then, b=1 or b=2?

or b is the source code? say, 14, 30, 21... if b is the source code, then there're many private keys.

any help will be appreciated。