Replace each character in string s1 with the character that is shifted 13 positions from it (wrap back to the beginning if necessary). So A ? N, B ? O, and so on until Y ? L and Z ? M. Case of the output should match the case of the input, so a ? n. Non-alphabetic characters are left in place and untouched.
s1 = 'I love MATLAB'
s2 = 'V ybir ZNGYNO'
See the Wikipedia article for more info.