#include <iostream>
#include <algorithm>
#include <cstring>
#include <vector>

using namespace std;

const int MAXN = 20;
const int MAXK = 2000000;

int n, m, sn;
long long k;
int a[MAXN][MAXN], back[MAXK];
vector<pair<long long, string> > v1, v2;
pair<string, pair<int, int> > srt[MAXK];

void generate (string currentStr, int lG, int rG, long long currentScore, vector<pair<long long, string> >& vec) {
    if (lG == rG + 1) {
        vec.push_back(make_pair(currentScore, currentStr));
        return ;
    } else {
        for(int i = 1; i <= n; i++)
            generate(currentStr + (char)('a' + (i - 1)), lG + 1, rG, currentScore + a[lG][i], vec);
    }
}

bool myCompare (const pair<long long, string>& self, const pair<long long, string>& other) {
    return (self.first > other.first || (self.first == other.first && self.second < other.second));
}

long long getGoodPairs (long long middle) {
    long long goodPairs = 0;
    int pe = (int)v2.size() - 1;
    for(int i = 0; i < v1.size(); i++) {
        while (pe > 0 && v2[pe].first + v1[i].first < middle)
            --pe;
        if (v2[pe].first + v1[i].first >= middle)
            goodPairs += pe + 1;
    }
    return goodPairs;
}

int main(int argc, const char * argv[]) {
    //freopen("input.txt", "r", stdin);
    cin >> n >> m >> k;
    for(int i = 1; i <= m; i++)
        for(int j = 1; j <= n; j++)
            cin >> a[i][j];
    
    generate("", 1, m / 2, 0LL, v1);
    generate("", m / 2 + 1, m, 0LL, v2);
    
    sort(v1.begin(), v1.end(), &myCompare);
    sort(v2.begin(), v2.end(), &myCompare);
    
    long long left = 0, right = 10000000000LL;
    while (left < right) {
        long long middle = (left + right + 1) >> 1;
        long long goodPairs = getGoodPairs(middle);
        if (goodPairs >= k)
            left = middle;
        else
            right = middle - 1;
    }
    
    k -= getGoodPairs(left + 1);
    
    back[0] = 0;
    for(int i = 1; i < v2.size(); i++)
        if (v2[i].first != v2[i - 1].first)
            back[i] = i;
        else
            back[i] = back[i - 1];
    
    int pe = (int)v2.size() - 1;
    for(int i = 0; i < v1.size(); i++) {
        while (pe > 0 && v2[pe].first + v1[i].first < left)
            --pe;
        if (v2[pe].first + v1[i].first == left) {
            int right = pe;
            int left = back[pe];
            srt[++sn] = make_pair(v1[i].second, make_pair(left, right));
        }
    }
    
    sort(srt + 1, srt + sn + 1);
    for(int i = 1; i <= sn; i++) {
        int left = srt[i].second.first;
        int right = srt[i].second.second;
        if (k > right - left + 1)
            k -= right - left + 1;
        else {
            cout << srt[i].first << v2[left + k - 1].second << endl;
            return 0;
        }
    }
    
    return 0;
}