#include <bits/stdc++.h>
using namespace std;
const int maxn = 51;
const int maxm = 26;
const int dir[4][2] = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
struct Position {
  int x, y;
};
Position st, en;
int n, m, k;
char mp[maxn][maxn];
bool hv[maxm], us[maxm];
int hc;
string ph, th;
int ml, tl;
int sd[maxn][maxn], ed[maxn][maxn];
queue<Position> qe;
bool Bound(Position p) { return p.x >= 0 && p.x < n && p.y >= 0 && p.y < m; }
void Bfs(Position s, int dis[maxn][maxn]) {
  int i, j;
  Position nw, nx;
  for (i = 0; i < n; i++) {
    for (j = 0; j < m; j++) dis[i][j] = -1;
  }
  dis[s.x][s.y] = 0;
  qe.push(s);
  while (!qe.empty()) {
    nw = qe.front();
    qe.pop();
    for (i = 0; i < 4; i++) {
      nx.x = nw.x + dir[i][0];
      nx.y = nw.y + dir[i][1];
      if (Bound(nx) && dis[nx.x][nx.y] == -1) {
        if (!(mp[nx.x][nx.y] >= 'a' && mp[nx.x][nx.y] <= 'z' &&
              us[mp[nx.x][nx.y] - 'a'] == 0)) {
          qe.push(nx);
          dis[nx.x][nx.y] = dis[nw.x][nw.y] + 1;
        }
      }
    }
  }
}
void FindPath() {
  vector<Position> tmp;
  Position nw, nx;
  char mi = 'z' + 1;
  int step = 1;
  int i;
  qe.push(st);
  while (!qe.empty()) {
    nw = qe.front();
    qe.pop();
    for (i = 0; i < 4; i++) {
      nx.x = nw.x + dir[i][0];
      nx.y = nw.y + dir[i][1];
      if (Bound(nx) && sd[nx.x][nx.y] == step &&
          sd[nx.x][nx.y] + ed[nx.x][nx.y] == tl) {
        sd[nx.x][nx.y] = -1;
        if (mp[nx.x][nx.y] < mi) {
          mi = mp[nx.x][nx.y];
          tmp.clear();
          tmp.push_back(nx);
        } else if (mp[nx.x][nx.y] == mi) {
          tmp.push_back(nx);
        }
      }
    }
    if (qe.empty()) {
      if (tmp.empty()) break;
      while (!tmp.empty()) {
        qe.push(tmp.back());
        tmp.pop_back();
      }
      if (mi != 'T') th += mi;
      mi = 'z' + 1;
      step++;
    }
  }
}
void Run() {
  tl = -1;
  th = "";
  Bfs(st, sd);
  Bfs(en, ed);
  if (sd[en.x][en.y] == -1) return;
  tl = sd[en.x][en.y];
  FindPath();
  if (ml == -1 || ml > tl || (ml == tl && ph > th)) {
    ml = tl;
    ph = th;
  }
}
void Select(int xp, int au) {
  if (au >= k) {
    Run();
    return;
  }
  if (xp >= maxm) {
    return;
  }
  if (hv[xp]) {
    us[xp] = 1;
    Select(xp + 1, au + 1);
    us[xp] = 0;
  }
  Select(xp + 1, au);
}
int main() {
  int i, j;
  while (cin >> n >> m >> k) {
    for (i = 0; i < n; i++) cin >> mp[i];
    memset(hv, 0, sizeof(hv));
    for (i = 0; i < n; i++) {
      for (j = 0; j < m; j++) {
        if (mp[i][j] >= 'a' && mp[i][j] <= 'z')
          hv[mp[i][j] - 'a'] = 1;
        else if (mp[i][j] == 'S')
          st.x = i, st.y = j;
        else if (mp[i][j] == 'T')
          en.x = i, en.y = j;
      }
    }
    hc = 0;
    for (i = 0; i < maxm; i++)
      if (hv[i]) hc++;
    ml = -1;
    ph = "";
    if (k >= hc) {
      for (i = 0; i < maxm; i++) us[i] = 1;
      Run();
    } else {
      memset(us, 0, sizeof(us));
      Select(0, 0);
    }
    if (ml == -1)
      cout << "-1" << endl;
    else
      cout << ph << endl;
  }
  return 0;
}
