Any alternate way to execute the program faster

Question

n,k,m=map(int,input().split())
s=input()
while m>0:
    pre=len(s)
    for i in s:
        s+=str(int(i)*k)
    s=s[pre:]
    m-=1
print(len(s))

Aim is to determine the number of white balls after M levels. Note that K remains the same for every level.

n,m,k are integers and s is string called power of ball(0<=s>=9) , s is single digit

INPUT-4 9 2
5418
OUTPUT-14
Exlanation-Initially S=5418
After the 1st, level S=4536972
After the 2nd level, S=36452754816318
length of s is 14 

Answer 1

n, k, m = map(int, input().split())
s = input()
for m in range(m, 0, -1):
    pre = len(s)
    for i in s:
        s += str(int(i) * k)
    s = s[pre:]
print(len(s))

Code above has O(m*s) time Complexity