Go: package main

import (
    "bufio"
    "os"
    "fmt"
)

var sc = bufio.NewScanner(os.Stdin)

/**************  Cell  *****************/
type Cell struct {
    item interface{}
    next *Cell
    prev *Cell
}

func NewCell(x interface{}, ne *Cell, pr *Cell) *Cell {
    newCp := new(Cell)
    newCp.item = x
    newCp.next = ne
    newCp.prev = pr
    return newCp
}
/************  End Cell  ***************/

/**************  Queue  ****************/
type Queue struct {
    size int
    front *Cell
    rear *Cell
}

func NewQueue() *Queue {
    return new(Queue)
}

func (q *Queue) Push(x interface{}) {
    newNode := NewCell(x, nil, nil)
    if q.size == 0 {
        q.front = newNode
        q.rear = newNode
    } else {
        newNode.prev = q.rear
        q.rear.next = newNode
        q.rear = newNode
    }
    q.size++
}

func (q *Queue) PopLeft() interface{} {
    if q.size == 0 {
        return nil
    } else {
        x := q.front.item
        if q.size == 1 {
            q.front = nil
            q.rear = nil
        } else {
            q.front = q.front.next
            q.front.prev = nil
        }
        q.size--
        return x
    }
}

func (q *Queue) IsEmpty() bool {
    return q.size == 0
}

func (q *Queue) Length() int {
    return q.size
}

type State struct {
    color string
    cnt int
}

func main() {
    for {
        sc.Scan()
        init := sc.Text()
        if init == "0" {
            break
        }
        bfs(init)
    }
}

func bfs(init string) {
    dic := make(map[string]int)
    que := NewQueue()
    que.Push(State{init, 0})
    length := len(init)
    isSolved := false
    for !que.IsEmpty() {
        state := que.PopLeft().(State)
        _, ok := dic[state.color]
        if ok {
            continue
        } else {
            dic[state.color] = state.cnt
        }

        if isSingleColor(state.color) {
            fmt.Println(state.cnt)
            isSolved = true
            break
        }

        for i := 0;i < length - 1; i++ {
            color1, color2 := string(state.color[i]), string(state.color[i + 1])
            if color1 != color2 {
                color3 := other(color1, color2)
                if i <= length - 2 {
                    que.Push(State{state.color[:i] + color3 + state.color[i + 2:], state.cnt + 1})
                } else {
                    que.Push(State{state.color[:i] + color3, state.cnt + 1})
                }
            }
        }
    }
    if !isSolved {
        fmt.Println("NA")
    }
}

func other(color1, color2 string) string {
    if color1 == "r" {
        if color2 == "g" {
            return "bb"
        } else if color2 == "b" {
            return "gg"
        }
    } else if color1 == "g" {
        if color2 == "r" {
            return "bb"
        } else if color2 == "b" {
            return "rr"
        }
    } else if color1 == "b" {
        if color2 == "r" {
            return "gg"
        } else if color2 == "g" {
            return "rr"
        }
    }
    return ""
}

func isSingleColor(color string) bool {
    for _, c := range color {
        if string(c) != string(color[0]) {
            return false
        }
    }
    return true
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <map>
#include <queue>
#include <cstring>
#include <sstream>
using namespace std;

int ar[11], ag[11], ab[11];
bool checked[59049];
int p3[11];
map<int,int> change = {
    {0,0}, {1,8}, {2,4}, {3,8}, {4,4}, {5,0}, {6,4}, {7,0}, {8,8}
};

int conv(string s) {
    int ret = 0;
    for (int i=0; i<(int)s.size(); ++i) {
        ret *= 3;
        if (s[i] == 'g') ret += 1;
        if (s[i] == 'b') ret += 2;
    }
    return ret;
}

int main() {
    p3[0] = 1;
    for (int i=1; i<=10; ++i) {
        p3[i] = p3[i-1] * 3;
    }
    ar[0] = ag[0] = ab[0] = 0;
    for (int i=1; i<=10; ++i) {
        ar[i] = ar[i-1];
        ag[i] = ag[i-1] * 3 + 1;
        ab[i] = ab[i-1] * 3 + 2;
    }
    string s;
    while (cin >> s, s != "0") {
        memset(checked, false, sizeof checked);
        int n = s.size();
        int first = conv(s);
        queue<pair<int,int> > q;
        q.push(make_pair(first, 0));
        checked[first] = true;
        int res = -1;
        while ( ! q.empty()) {
            int col = q.front().first;
            int t = q.front().second;
            q.pop();
            if (col == ar[n] || col == ag[n] || col == ab[n]) {
                res = t;
                break;
            }
            for (int i=0; i<n-1; ++i) {
                int replace = change[(col / p3[i]) % 9];
                int nx = (col / p3[i+2]) * p3[i+2] + replace * p3[i] + (col % p3[i]);
                if ( ! checked[nx]) {
                    q.push(make_pair(nx, t+1));
                    checked[nx] = true;
                }
            }
        }
        if (res < 0) {
            cout << "NA" << endl;
        } else {
            cout << res << endl;
        }
    }
    return 0;
}

C++: 
#include <cstdio>
#include <iostream>
#include <vector>
#include <map>
#include <set>
#include <string>
#include <cstring>
#include <sstream>
#include <algorithm>
#include <functional>
#include <queue>
#include <stack>
#include <cmath>
#include <iomanip>
 
using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n)  FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define CLR(a) memset((a), 0 ,sizeof(a))
#define MCLR(a) memset((a), -1 ,sizeof(a))
#define SZ(a) (sizeof(a))
#define FILL(a,v) fill((int*)a,(int*)(a+(SZ(a)/SZ(*(a)))),v)
#define dump(x)  cerr << #x << " = " << (x) << endl

bool cheak(int x,int y,int xMax,int yMax)
{return x >= 0 && y >= 0 && xMax > x && yMax > y;}
const int dx[4]={-1,0,1,0},dy[4]={0,1,0,-1};
const int INF = 2147483647;
typedef pair<int,int> P;
typedef long long ll;
typedef unsigned long long ull;

//???????????¢?´¢
struct Bfs
{
	set<string> memo;
	set<string> memo2;
	int search(string s)
	{
		queue<string> que;
		queue<int>    time;
		que.push(s);
		time.push(0);
		memo.insert(s);
		memo2.insert(s);
		while(que.size())
		{
			string now = que.front();que.pop();
			int anstime = time.front();time.pop();
			//??????
			
			bool end = true;
			REP(i,now.size()-1)
			if(now[i] != now[i+1])
			end = false;
			if(end)return anstime; 
			//

			REP(i,now.size()-1)
			{
				string ts = now;
				char c0 = now[i];
				char c1 = now[i+1];
				char sc;
				if(c0 != c1)
				{
					if(c0 != 'b' && c1 != 'b')
						sc = 'b';
					if(c0 != 'r' && c1 != 'r')
						sc = 'r';
					if(c0 != 'g' && c1 != 'g')
						sc = 'g';
					
					ts[i] = sc;ts[i+1] = sc;
					if(!memo2.count(ts))
					{
						if(memo.count(ts))
							memo2.insert(ts);
						memo.insert(ts);
						que.push(ts);
						time.push(1+anstime);
					}
				}
			}
		}
		return -1;
	}
};

int main()
{
	string str;
	while(cin >> str && str != "0")
	{
		Bfs bfs;
		int ans = bfs.search(str);
		if(ans == -1)
			cout << "NA" << endl;
		else
			cout <<  ans << endl;
	}
	return 0;
}


//????¬????????????????????????¨?????????
/*
	REP(i,mx)
	{
		REP(j,my)
		cout << n[i][j];
		cout << endl;
	}
	cout << endl;
*/