string cipher(int k, string s) {
    int n = s.size();
    string res = "";
    res.push_back(s[0]);
    
    for(int i=1;i<n-k+1;i++){
        if(i < k){
            int x = (int)(s[i]-'0') ^ (int)(s[i-1]-'0');
            char ch = ('0'+x);
            res.push_back(ch);
        }
        else{
            int r = res.size();
            int x = 0;
            for(int j=r-1;j>=r-k+1;j--){
                x ^= (int)(res[j]-'0');
            }
            x ^= (int)(s[i]-'0');
            char ch = (char)('0'+x);
            res.push_back(ch);
        }
    }
    return res;
}

def cipher(k, s):
    # Write your code here
    if(k==0):
        return s
    S=s[0]
    for i in range(1,len(s)-(k-1)):
        S+=str(int(s[i])^int(s[i-1])^(0 if i<k else int(S[i-k])))         
    return S

string cipher(int k, string s)
{
  int n = (int)s.size(), m = n-k+1, t = k+1;
  string r(m, 0), x(t, 0);
  for (int i = 0; i < m; i++) {
    x[i%t] = x[(i+1)%t] ^ (s[n-1-i]-'0');
    r[m-1-i] = (x[i%t] ^ x[(i+k)%t]) + '0';
  }
  return r;
}