def count_c(s, c):
    if len(s)==0:
        return
    if s[0]==c:
        return 1 + count_c(s[1:], c)
    else:
        return 0 + count_c(s[1:], c)

print('the number of letters is: ' + count_c('ciao amico come cacchio stai?','c'))
print('the number of letters is: ' + count_c('ccc','c'))