Python program where all occurrences of different char have been changed to '$' except first of that character

a = input('Enter a string ')
b = a
nc = ''
for n in a:
    c=0
    for m in b:
        if(m is n and c>0):
            nc = nc + '$'
        elif(m is n):
            c=c+1
            nc = nc + m
        else:
            nc = nc + m
    b = nc
    nc = ''
print(b)