#include <bits/stdc++.h>
using namespace std;
string res;
int c[26], w[26];
bool vis[26];
int f[60][60];
char g[60][60];
int n, m, k, s1, s2, e1, e2, tot, step, maxstep;
int way[4][2] = {{0, -1}, {0, 1}, {1, 0}, {-1, 0}};
struct In {
  int l;
  int r;
  string ss;
} s[3000];
void bfs() {
  memset(f, -1, sizeof(f));
  int i, j, first = 0, last = 1, len = 1, move = 1, ll;
  s[0].l = s1;
  s[0].r = s2;
  s[0].ss = "";
  f[s1][s2] = 0;
  string a;
  bool z = false;
  while (first < last) {
    for (i = first; i < last; i++) {
      for (j = 0; j < 4; j++) {
        int p, q;
        p = s[i].l + way[j][0];
        q = s[i].r + way[j][1];
        if (p >= 0 && p < n && q >= 0 && q < m) {
          if (f[p][q] != -1 && f[p][q] < move) continue;
          a = s[i].ss;
          if (step == maxstep && a >= res) continue;
          if (g[p][q] == 'T') {
            if (res == "") {
              res = a;
              step = move;
            } else {
              int x1 = a.length(), x2 = res.length();
              if (x1 < x2 || (x1 == x2 && a < res)) {
                res = a;
                if (move < step) step = move;
              }
            }
            z = true;
          } else if (!z) {
            bool zz = false;
            for (ll = 0; ll < tot && ll < k; ll++) {
              if (w[ll] == g[p][q] - 'a') {
                zz = true;
                break;
              }
            }
            if (zz) {
              a += g[p][q];
              for (ll = last; ll < len; ll++)
                if (s[ll].l == p && s[ll].r == q) {
                  if (s[ll].ss > a) s[ll].ss = a;
                  break;
                }
              if (ll == len) {
                s[len].l = p;
                s[len].r = q;
                s[len].ss = a;
                len++;
                f[p][q] = move;
              }
            }
          }
        }
      }
    }
    if (z) break;
    first = last;
    last = len;
    move++;
    if (move > step) break;
  }
}
void dfs(int x, int y) {
  if (x == tot) return;
  w[y] = c[x];
  if (y == k - 1) {
    bfs();
  } else
    dfs(x + 1, y + 1);
  dfs(x + 1, y);
}
int main() {
  while (cin >> n >> m >> k) {
    int i, j;
    tot = 0;
    memset(vis, 0, sizeof(vis));
    for (i = 0; i < n; i++) {
      cin >> g[i];
      for (j = 0; j < m; j++) {
        if (g[i][j] == 'S') {
          s1 = i;
          s2 = j;
        } else if (g[i][j] == 'T') {
          e1 = i;
          e2 = j;
        } else if (g[i][j] >= 'a' && g[i][j] <= 'z') {
          if (!vis[g[i][j] - 'a']) {
            c[tot++] = g[i][j] - 'a';
            vis[g[i][j] - 'a'] = 1;
          }
        }
      }
    }
    maxstep = abs(s1 - e1) + abs(s2 - e2);
    step = 3000;
    if (tot <= k) {
      for (i = 0; i < tot; i++) w[i] = c[i];
      bfs();
    } else
      dfs(0, 0);
    if (step == 3000)
      cout << -1 << endl;
    else
      cout << res << endl;
  }
  return 0;
}
