#include <bits/stdc++.h>
using namespace std;
int n, m;
int k;
char str[55][55];
int sr, sc;
int er, ec;
string cur;
bool can[26];
int am = 0;
int mark[55][55];
int dist[55][55];
int dx[] = {1, -1, 0, 0};
int dy[] = {0, 0, 1, -1};
int qhead, qtail;
int q[55 * 55 * 4];
int nqtail;
int nq[55 * 55 * 4];
int go(int r, int c) {
  qhead = qtail = 0;
  mark[r][c] = am;
  dist[r][c] = 0;
  q[qtail++] = r;
  q[qtail++] = c;
  while (qhead < qtail) {
    int cr = q[qhead++];
    int cc = q[qhead++];
    if (str[cr][cc] == 'S') return dist[cr][cc];
    for (int dir = 0; dir < 4; ++dir) {
      int nr = cr + dx[dir];
      int nc = cc + dy[dir];
      if (nr < 0 || nr >= n || nc < 0 || nc >= m) continue;
      if (mark[nr][nc] == am) continue;
      if (str[nr][nc] != 'S' && (str[nr][nc] == 'T' || !can[str[nr][nc] - 'a']))
        continue;
      mark[nr][nc] = am;
      dist[nr][nc] = dist[cr][cc] + 1;
      q[qtail++] = nr;
      q[qtail++] = nc;
    }
  }
  return 987654321;
}
void rec(int r, int c) {
  qhead = qtail = 0;
  nqtail = 0;
  q[qtail++] = r;
  q[qtail++] = c;
  mark[r][c] = am;
  cur = "";
  for (int stp = 0; stp < dist[r][c] - 1; ++stp) {
    int best = 255;
    for (int i = 0; i < qtail; i += 2) {
      int cr = q[i];
      int cc = q[i + 1];
      for (int dir = 0; dir < 4; ++dir) {
        int nr = cr + dx[dir];
        int nc = cc + dy[dir];
        if (mark[nr][nc] == am) continue;
        if (nr < 0 || nr >= n || nc < 0 || nc >= m) continue;
        if (dist[nr][nc] != dist[cr][cc] - 1) continue;
        if (str[nr][nc] != 'T' &&
            (str[nr][nc] == 'S' || !can[str[nr][nc] - 'a']))
          continue;
        best = min(best, (int)str[nr][nc]);
      }
    }
    cur += char(best);
    for (int i = 0; i < qtail; i += 2) {
      int cr = q[i];
      int cc = q[i + 1];
      for (int dir = 0; dir < 4; ++dir) {
        int nr = cr + dx[dir];
        int nc = cc + dy[dir];
        if (mark[nr][nc] == am) continue;
        if (nr < 0 || nr >= n || nc < 0 || nc >= m) continue;
        if (dist[nr][nc] != dist[cr][cc] - 1) continue;
        if (str[nr][nc] != 'T' &&
            (str[nr][nc] == 'S' || !can[str[nr][nc] - 'a']))
          continue;
        if (best == str[nr][nc]) {
          mark[nr][nc] = am;
          nq[nqtail++] = nr;
          nq[nqtail++] = nc;
        }
      }
    }
    for (int i = 0; i < nqtail; ++i) {
      q[i] = nq[i];
    }
    qtail = nqtail;
    nqtail = 0;
  }
}
int main() {
  cin >> n >> m >> k;
  for (int i = 0; i < n; ++i) {
    cin >> str[i];
    for (int j = 0; j < m; ++j)
      if (str[i][j] == 'S') {
        sr = i;
        sc = j;
      } else if (str[i][j] == 'T') {
        er = i;
        ec = j;
      }
  }
  int dist = 987654321;
  string res = "";
  memset(can, 0, sizeof(can));
  memset(mark, -1, sizeof(mark));
  for (int a = 0; a < 26; ++a) {
    for (int b = a; b < 26; ++b) {
      for (int c = b; c < 26; ++c) {
        for (int d = c; d < 26; ++d) {
          int cnt = 1;
          if (b != a) ++cnt;
          if (c != b) ++cnt;
          if (d != c) ++cnt;
          if (cnt > k) continue;
          ++am;
          cur = "";
          can[a] = 1;
          can[b] = 1;
          can[c] = 1;
          can[d] = 1;
          int ndist = go(er, ec);
          if (ndist <= dist && ndist != 987654321) {
            ++am;
            rec(sr, sc);
            if (ndist < dist || ndist == dist && cur < res) {
              dist = ndist;
              res = cur;
            }
          }
          can[a] = 0;
          can[b] = 0;
          can[c] = 0;
          can[d] = 0;
        }
      }
    }
  }
  if (dist == 987654321)
    cout << "-1" << endl;
  else
    cout << res << endl;
  return 0;
}
