#include <bits/stdc++.h>
using namespace std;
int R, C, K, Tail;
int Board[60][60], Road[60][60], Best[60][60], Oki[60][60];
int Allow[100000][5];
int Last[60][60];
char Now[5];
int Res;
int Check[30];
char Ans[10000];
int QR[10000], QC[10000];
void pre_process(char now, int left) {
  if (now > 'z') {
    if (left == 0) {
      int i;
      for (i = 0; i < (int)(K); i++) Allow[Tail][i] = (int)(Now[i] - 'a');
      Tail++;
    }
    return;
  }
  pre_process(now + 1, left);
  if (left > 0) {
    Now[left - 1] = now;
    pre_process(now + 1, left - 1);
  }
}
bool isValid(int r, int c) {
  return 0 <= r && r < R && 0 <= c && c < C && Road[r][c];
}
int main() {
  int i, j, k, r, c;
  int sr, sc, er, ec;
  char a;
  int dr[4] = {-1, 0, 1, 0};
  int dc[4] = {0, 1, 0, -1};
  int tt, et;
  scanf("%d%d%d", &R, &C, &K);
  for (i = 0; i < (int)(R); i++) {
    for (j = 0; j < (int)(C); j++) {
      scanf("%c", &a);
      if (!('a' <= a && a <= 'z') && a != 'S' && a != 'T') {
        j--;
        continue;
      }
      if ('a' <= a && a <= 'z')
        Board[i][j] = a - 'a';
      else if (a == 'S')
        Board[i][j] = -1, sr = i, sc = j;
      else
        Board[i][j] = -2, er = i, ec = j;
    }
  }
  Tail = 0;
  pre_process('a', K);
  Res = -1;
  for (i = 0; i < (int)(Tail); i++) {
    for (j = 0; j < (int)(26); j++) Check[j] = 0;
    for (j = 0; j < (int)(K); j++) Check[Allow[i][j]] = 1;
    for (r = 0; r < (int)(R); r++)
      for (c = 0; c < (int)(C); c++) {
        if (Board[r][c] >= 0)
          Road[r][c] = Check[Board[r][c]];
        else
          Road[r][c] = 1;
        Best[r][c] = -1;
        Oki[r][c] = 0;
        Last[r][c] = 0;
      }
    tt = 1;
    QR[0] = sr, QC[0] = sc;
    Best[sr][sc] = 0;
    for (j = 0; j < (int)(tt); j++) {
      r = QR[j], c = QC[j];
      for (k = 0; k < (int)(4); k++)
        if (isValid(r + dr[k], c + dc[k]) && Best[r + dr[k]][c + dc[k]] == -1) {
          Best[r + dr[k]][c + dc[k]] = Best[r][c] + 1;
          QR[tt] = r + dr[k], QC[tt] = c + dc[k];
          tt++;
        }
    }
    if (Best[er][ec] == -1 || (Res != -1 && Best[er][ec] > Res)) continue;
    bool update = false;
    if (Res == -1 || Best[er][ec] < Res) update = true;
    Res = Best[er][ec];
    Ans[Res - 1] = Ans[Res] = 0;
    tt = 1;
    QR[0] = er, QC[0] = ec;
    Oki[er][ec] = 1;
    for (j = 0; j < (int)(tt); j++) {
      r = QR[j], c = QC[j];
      for (k = 0; k < (int)(4); k++)
        if (isValid(r + dr[k], c + dc[k]) &&
            Best[r + dr[k]][c + dc[k]] == Best[r][c] - 1 &&
            !Oki[r + dr[k]][c + dc[k]]) {
          Oki[r + dr[k]][c + dc[k]] = 1;
          QR[tt] = r + dr[k], QC[tt] = c + dc[k];
          tt++;
        }
    }
    int cur = 0, tt = 1, move = 0;
    QR[0] = sr, QC[0] = sc;
    Best[sr][sc] = 0;
    while (cur < Res - 1) {
      et = tt;
      int best = -1;
      for (j = move; j < et; j++) {
        r = QR[j], c = QC[j];
        for (k = 0; k < (int)(4); k++)
          if (isValid(r + dr[k], c + dc[k]) && Oki[r + dr[k]][c + dc[k]] &&
              Best[r + dr[k]][c + dc[k]] == cur + 1) {
            if (best == -1 || best > Board[r + dr[k]][c + dc[k]]) {
              best = Board[r + dr[k]][c + dc[k]];
            }
          }
      }
      if (!update && (char)(best + 'a') > Ans[cur]) break;
      if (!update && (char)(best + 'a') < Ans[cur]) update = true;
      Ans[cur] = (char)(best + 'a');
      cur++;
      for (j = move; j < et; j++) {
        r = QR[j], c = QC[j];
        for (k = 0; k < (int)(4); k++)
          if (isValid(r + dr[k], c + dc[k]) && Oki[r + dr[k]][c + dc[k]] &&
              Best[r + dr[k]][c + dc[k]] == cur &&
              !Last[r + dr[k]][c + dc[k]]) {
            if (best == Board[r + dr[k]][c + dc[k]]) {
              Last[r + dr[k]][c + dc[k]] = 1;
              QR[tt] = r + dr[k], QC[tt] = c + dc[k];
              tt++;
            }
          }
      }
      move = et;
    }
  }
  if (Res == -1)
    printf("%d\n", Res);
  else
    printf("%s\n", Ans);
  return 0;
}
