#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:60777216")
using namespace std;
long long gcd(long long a, long long b) { return a == 0 ? b : gcd(b % a, a); }
string filename = "test";
int n, m, k;
vector<string> mapa;
int dx[] = {-1, 0, 1, 0};
int dy[] = {0, 1, 0, -1};
bool allowed[26];
int ctoi(char c) { return c - 'a'; }
int valid(int i, int j) { return i >= 0 && i < n && j >= 0 && j < m; }
pair<int, int> Q[100000];
int D[51][51];
int main() {
  scanf("%d %d %d", &n, &m, &k);
  string tmp_str;
  for (int _n(n), i(0); i < _n; i++) {
    cin >> tmp_str;
    mapa.push_back(tmp_str);
  }
  pair<int, int> start, end;
  vector<char> all_letters;
  for (int _n(n), i(0); i < _n; i++)
    for (int _n(m), j(0); j < _n; j++) {
      if (mapa[i][j] == 'S') {
        start = pair<int, int>(i, j);
      } else if (mapa[i][j] == 'T') {
        end = pair<int, int>(i, j);
      } else {
        all_letters.push_back(mapa[i][j]);
      }
    }
  for (int _n(4), dir(0); dir < _n; dir++) {
    int nx = start.first + dx[dir], ny = start.second + dy[dir];
    if (valid(nx, ny) && pair<int, int>(nx, ny) == end) {
      cout << "\n";
      return 0;
    }
  }
  sort(((all_letters)).begin(), ((all_letters)).end()),
      (all_letters)
          .erase(unique(((all_letters)).begin(), ((all_letters)).end()),
                 (all_letters).end());
  int nn = all_letters.size();
  int ires = -1;
  string sres = "";
  for (int _n(k + 1), kk(1); kk < _n; kk++) {
    if (kk > all_letters.size()) break;
    unsigned int mask = (1 << kk) - 1;
    while (!(mask & (1 << nn))) {
      memset(allowed, false, sizeof(allowed));
      for (int _n(nn), i(0); i < _n; i++)
        if (mask & (1 << i)) {
          allowed[ctoi(all_letters[i])] = true;
        }
      int wr = 0, rd = 0;
      memset(D, -1, sizeof(D));
      Q[wr++] = end;
      D[end.first][end.second] = 0;
      while (rd < wr) {
        pair<int, int> now = Q[rd++];
        for (int _n(4), dir(0); dir < _n; dir++) {
          int nx = now.first + dx[dir], ny = now.second + dy[dir];
          if (!valid(nx, ny)) continue;
          if (D[nx][ny] != -1) continue;
          if (mapa[nx][ny] != 'T' && mapa[nx][ny] != 'S' &&
              !allowed[ctoi(mapa[nx][ny])])
            continue;
          D[nx][ny] = D[now.first][now.second] + 1;
          Q[wr++] = pair<int, int>(nx, ny);
        }
      }
      int ires_now = D[start.first][start.second];
      if (ires_now != -1 && !(ires != -1 && ires_now > ires)) {
        string sres_now = "";
        vector<pair<int, int> > pos[2];
        int cur_index = 0;
        pos[cur_index].push_back(start);
        int d = D[start.first][start.second];
        while (d != 0) {
          sres_now.push_back(
              mapa[pos[cur_index][0].first][pos[cur_index][0].second]);
          int next_index = cur_index ^ 1;
          bool exit_found = false;
          char min_c = (char)('z' + 1);
          for (int _n(pos[cur_index].size()), i(0); i < _n; i++) {
            for (int _n(4), dir(0); dir < _n; dir++) {
              int nx = pos[cur_index][i].first + dx[dir],
                  ny = pos[cur_index][i].second + dy[dir];
              if (!valid(nx, ny)) continue;
              if (D[nx][ny] == -1) continue;
              if (D[nx][ny] + 1 != d) continue;
              if (mapa[nx][ny] == 'T') {
                exit_found = true;
                break;
              }
              D[nx][ny] = -1;
              if (mapa[nx][ny] < min_c) {
                min_c = mapa[nx][ny];
                pos[next_index].clear();
                pos[next_index].push_back(pair<int, int>(nx, ny));
              } else if (mapa[nx][ny] == min_c) {
                pos[next_index].push_back(pair<int, int>(nx, ny));
              }
            }
            if (exit_found) break;
          }
          if (exit_found) break;
          --d;
          cur_index = next_index;
        }
        if (ires == -1 || ires_now < ires) {
          ires = ires_now;
          sres = sres_now;
        } else if (ires_now == ires && sres_now < sres) {
          ires = ires_now;
          sres = sres_now;
        }
      }
      int lo = mask & ~(mask - 1);
      int lz = (mask + lo) & ~mask;
      mask |= lz;
      mask &= ~(lz - 1);
      mask |= (lz / lo / 2) - 1;
    }
  }
  if (ires == -1) {
    printf("-1\n");
  } else {
    cout << sres.substr(1) << endl;
  }
  return 0;
}
