Decrypt

input/code.cpp: In function 'int main()':
input/code.cpp:36:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i=0; i<f.size(); ++i){
                          ^
input/code.cpp:42:26: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i=0; i<S.size(); ++i){
                          ^

#include <iostream>
#include <vector>
using namespace std;
string f;
string S;
int k;

const int INF=1222333444;

vector<int> bck[30];


int u[101010];

char mp[265];

int loop;

void gbck(int c, int lp){
  mp[lp]=c+'a';
  if (u[c]){
    loop=lp;
    return;
  }
  u[c]=1;
  
  if (bck[c].size()!=1) loop=-lp;
  else  		gbck(bck[c][0], lp+1);
  
  u[c]=0;
}


int main(){
  cin >> f >> k;
  for (int i=0; i<f.size(); ++i){
    bck[f[i]-'a'].push_back(i);
  }
  
  cin >> S;
  string ans="";
  for (int i=0; i<S.size(); ++i){
    gbck(S[i]-'a', 0);
    if (loop<=0){
      if (-loop<k){
	cout << "impossible\n";
	return 0;
      }else{
	ans+=mp[k];
      }
    }else{
      ans+=mp[k%loop];
    }
  }
  cout << ans << "\n";
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: ACCEPTED

Test 25

Verdict: ACCEPTED

Test 26

Verdict: ACCEPTED

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED

Test 29

Verdict: ACCEPTED

Test 30

Verdict: ACCEPTED