#include <bits/stdc++.h>
using namespace std;
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
int query(string q);
vector<int> rem(26);
string go(const string& pre, const string& post, int N, int S) {
if (S < 0) return "";
int cnt = 0;
string mid(1, 'a' + S);
if (rem[S] && query(pre + mid + post) == sz(pre) + 1 + sz(post)) {
int hi = sz(pre) + sz(post) + rem[S] + 1;
int lo = sz(pre) + sz(post) + 1;
while (hi - lo > 1) {
int mid = (lo + hi) / 2;
cerr << "test tot length " << mid << " of char" << endl;
string ask = pre;
rep(i,0,mid - sz(pre) - sz(post)) ask.push_back('a' + S);
ask.insert(ask.end(), post.begin(), post.end());
int cnt = query(ask);
if (cnt >= mid) lo = mid;
else hi = mid;
}
cnt = lo - sz(pre) - sz(post);
}
cerr << "with pre " << pre << " and post " << post << " at " << (char)(S+'a') << " we found " << cnt << endl;
string res;
rep(i,0,cnt + 1) {
string npre = pre;
rep(j,0,i) npre.push_back('a' + S);
string npost;
rep(j,0,cnt-i) npost.push_back('a' + S);
npost += post;
res += go(npre, npost, N - sz(res) + i, S - 1);
if (i != cnt) {
res.push_back('a' + S);
rem[S]--;
}
}
return res;
}
string guess(int N, int S) {
rep(i,0,S) {
string s(N, 'a' + i);
rem[i] = query(s);
}
return go("", "", N, S - 1);
}
