#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <queue>
#include <stack>
#include <algorithm>

using namespace std;

#define rep(i,j) REP((i), 0, (j))
#define REP(i,j,k) for(int i=(j);(i)<(k);++i)
#define BW(a,x,b) ((a)<=(x)&&(x)<=(b))
#define ALL(v) (v).begin(), (v).end()
#define LENGTHOF(x) (sizeof(x) / sizeof(*(x)))
#define AFILL(a, b) fill((int*)a, (int*)(a + LENGTHOF(a)), b)
#define MP make_pair
#define PB push_back
#define F first
#define S second
#define INF 1 << 30
#define EPS 1e-10

typedef pair<int, int> pi;
typedef pair<int, pi> pii;
typedef vector<int> vi;
typedef queue<int> qi;
typedef long long ll;

int X, Y;
int dx[] = {-1,0,1,0}, dy[] = {0,-1,0,1};
int res;
int closed[16][16];

void filling(int s[16][16], int y, int x, int from, int to){
  s[y][x] = to;
  rep(d, 4){
    int ny = y + dy[d], nx = x + dx[d];
    if(ny < 0 || ny >= Y || nx < 0 || nx >= X || s[ny][nx] != from) continue;
    filling(s, ny, nx, from, to);
  }
  return;
}

int check(int s[16][16]){
  int p = s[0][0];
  rep(i, Y) rep(j, X){
    if(s[i][j] != p) return 0;
  }
  return 1;
}

void next(int from[16][16], int to1[16][16], int to2[16][16]){
  int c1, c2;
  c1 = c2 = from[0][0];
  c1 += 1; c2 += 2;
  c1 %= 3; c2 %= 3;

  rep(i, Y) rep(j, X){
    to1[i][j] = to2[i][j] = from[i][j];
  }

  queue<pi>open;
  open.push(pi(0, 0));
  memset(closed, 0, sizeof(closed));
  to1[0][0] = c1; to2[0][0] = c2;

  while(!open.empty()){
    pi p = open.front();
    open.pop();

    rep(d, 4){
      int ny = p.F + dy[d];
      int nx = p.S + dx[d];

      if(ny < 0 || nx < 0 || ny >= Y || nx >= X || closed[ny][nx] || from[0][0] != from[ny][nx]) continue;
      to1[ny][nx] = c1;
      to2[ny][nx] = c2;
      closed[ny][nx] = 1;
      open.push(pi(ny, nx));
    }
  }
}

void dfs(int s[16][16], int n){
  if(check(s)){
    res = n;
    return;
  }
  if(res <= n) return;

  int n1[16][16];
  int n2[16][16];

  next(s, n1, n2);
  dfs(n1, n+1);
  dfs(n2, n+1);
  
  return;
}

int main(){
  while(scanf("%d%d", &X, &Y) && X+Y){
    int s[16][16];
    getchar();
    rep(i, Y) rep(j, X){
      char c = getchar();
      switch(c){
      case 'R': s[i][j] = 0; break;
      case 'G': s[i][j] = 1; break;
      case 'B': s[i][j] = 2; break;
      }
      getchar();
    }
    
    res = 18;
    dfs(s, 0);
    printf("%d\n", res);
  }
  return 0;
}