c=int(input())
st=str(input())
import string
asc=string.ascii_letters
sum=0
result=""
for i in range(c):
    sum+=(int(ord(st[i]))-96)*(26**(c-i-1))
re=(26**c-sum)
for i in range(c):
    if i==c-1:
        result+=chr(int((re%(26**(c-i))-re%(26**(c-i-1)))/(26**(c-i-1)))+97)
    else:
        result+=chr(int((re%(26**(c-i))-re%(26**(c-i-1)))/(26**(c-i-1)))+98)
print(result)