#include <bits/stdc++.h>
using namespace std;
const int c[4][2] = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
bool f[30], vis[60][60];
char a[60][60];
int n, m, K, d[60][60];
string ans;
pair<int, int> S, T;
queue<pair<int, int> > Q;
vector<pair<int, int> > b;
void bfs() {
  memset(d, 63, sizeof(d));
  int inf = d[0][0];
  d[T.first][T.second] = 0;
  Q.push(T);
  while (!Q.empty()) {
    int x0 = Q.front().first, y0 = Q.front().second;
    Q.pop();
    for (int i = 0; i < 4; i++) {
      int x = x0 + c[i][0], y = y0 + c[i][1];
      if (x > 0 && x <= n && y > 0 && y <= m && d[x0][y0] + 1 < d[x][y] &&
          (a[x][y] == 'S' || f[a[x][y] - 'a'])) {
        d[x][y] = d[x0][y0] + 1;
        Q.push(make_pair(x, y));
      }
    }
  }
  if (d[S.first][S.second] == inf) return;
  string now;
  Q.push(S);
  memset(vis, 0, sizeof(vis));
  vis[S.first][S.second] = true;
  while (1) {
    char ch = 'z' + 1;
    b.clear();
    while (!Q.empty()) {
      int x0 = Q.front().first, y0 = Q.front().second;
      Q.pop();
      for (int i = 0; i < 4; i++) {
        int x = x0 + c[i][0], y = y0 + c[i][1];
        if (x > 0 && x <= n && y > 0 && y <= m && d[x0][y0] == d[x][y] + 1) {
          if (a[x][y] < ch) {
            b.clear();
            ch = a[x][y];
          }
          if (a[x][y] == ch && !vis[x][y]) b.push_back(make_pair(x, y));
          vis[x][y] = true;
        }
      }
    }
    if (b.size() == 0 || ch == 'T') break;
    now += ch;
    for (int i = 0; i < b.size(); i++) Q.push(b[i]);
  }
  if (now.size() < ans.size() || now.size() == ans.size() && now < ans)
    ans = now;
}
void dfs(int dep, int last) {
  if (dep > K) return;
  bfs();
  for (int i = last + 1; i < 26; i++) {
    f[i] = true;
    dfs(dep + 1, i);
    f[i] = false;
  }
}
int main() {
  scanf("%d%d%d", &n, &m, &K);
  for (int i = 1; i <= n; i++) scanf("%s", a[i] + 1);
  for (int i = 1; i <= n * m; i++) ans += 'z';
  ans += 'z';
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= m; j++) {
      if (a[i][j] == 'S') S = make_pair(i, j);
      if (a[i][j] == 'T') T = make_pair(i, j);
    }
  for (int i = 0; i < 26; i++) {
    f[i] = true;
    dfs(1, i);
    f[i] = false;
  }
  if (ans.size() == n * m + 1)
    printf("-1\n");
  else
    printf("%s\n", ans.c_str());
  return (0);
}
