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 <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <cmath>
#include <stack>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <list>
#include <iostream>
#include <sstream>
#include <climits>
#include <cfloat>
#include <complex>

using namespace std;

int d[1 << 22];
void p(int s, int n)
{
	for (int i = n-1; i >= 0; --i)
	{
		int t = (s & (3 << 2*i)) >> 2*i;
		if (t == 1)
			putchar('r');
		else if (t == 2)
			putchar('g');
		else if (t == 3)
			putchar('b');
		else
			putchar('*');
	}
}
int main()
{
	char buf[16];
	while (cin.getline(buf, sizeof(buf)), *buf != '0')
	{
		int n = strlen(buf);
		int initial = 0;
		for (int i = 0; i < n; ++i)
		{
			int t;
			if (buf[i] == 'r')
				t = 1;
			else if (buf[i] == 'g')
				t = 2;
			else
				t = 3;

			initial = (initial << 2) | t;
		}

		memset(d, -1, sizeof(d));
		d[initial] = 0;
		queue<int> state;
		state.push(initial);
		int ans = -1;
		while (!state.empty())
		{
			int s = state.front();
			state.pop();
			bool ok = true;
			for (int i = 0; i < n-1; ++i)
			{
				int r = (s & (3 << 2*i)) >> 2*i;
				int l = (s & (3 << 2*(i+1))) >> 2*(i+1);
				bool same = l == r;
				ok &= same;
				if (!same)
				{
					int color = -1;
					for (int j = 1; j <= 3; ++j)
						if (j != l && j != r)
							color = j;
					int next = s & ~(15 << 2*i) | (5*color << 2*i);
					if (d[next] == -1)
					{
						d[next] = d[s] + 1;
						state.push(next);
					}
				}
			}
			if (ok)
			{
				ans = d[s];
				break;
			}
		}
		if (ans != -1)
			printf("%d\n", ans);
		else
			puts("NA");
	}

	return 0;
}

C++: // clear time: 01:55
#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <algorithm>

using namespace std;
typedef pair<string, int> bug;

int isOK(string str) {
    // cout << "str:" << str << " " << str.size() << endl;
    for ( int i = 0; i < str.size(); i++ ) {
        if ( str[0] != str[i] ) return 0;
    }
    return 1;
}

char cc(char a, char b) {
    // 並び替え
    if ( b < a ) {
        char tmp = a;
        a = b;
        b = tmp;
    }

    if ( a == 'b' && b == 'g' ) {
        return 'r';
    } else if ( a == 'b' && b == 'r' ) {
        return 'g';
    } else {
        return 'b';
    }
}

int has(vector<string> &list, string str) {
    vector<string>::iterator sit = find(list.begin(), list.end(), str);
    return sit != list.end();
}

void wfs(string str) {
    int cnt = -1;
    string first = str;
    queue<bug> open;
    vector<string> closed;

    // 初期状態をプッシュ
    open.push( make_pair(str, 0) );

    closed.push_back(str);


    while(!open.empty()) {
        bug b = open.front(); open.pop();
        string cs = b.first;

        if ( isOK(cs) ) {
            cnt = b.second;
            break;
        }

        int c = 0;
        for ( int i = 1; i < cs.size(); i++ ) {
            if ( cs[i-1] != cs[i] ) {
                string tmp = cs;
                char nc = cc(cs[i-1], cs[i]);
                tmp[i-1] = nc;
                tmp[i] = nc;

                if ( has(closed, tmp) ) continue;
                open.push( make_pair(tmp, b.second+1) );
                closed.push_back(tmp);
                c++;
            }
        }
    }

    if ( cnt == -1 ) {
        cout << "NA" << endl;
    } else {
        cout << cnt << endl;
    }


}

int main() {
    string n;

    while ( cin >> n, n != "0" ) {
        wfs(n);
    }

    return 0;
}