Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

var scanner = bufio.NewScanner(os.Stdin)

func main() {
	table := [6][5]string{
		{"a", "b", "c", "d", "e"},
		{"f", "g", "h", "i", "j"},
		{"k", "l", "m", "n", "o"},
		{"p", "q", "r", "s", "t"},
		{"u", "v", "w", "x", "y"},
		{"z", ".", "?", "!", " "},
	}
	for scanner.Scan() {
		input := strings.Split(scanner.Text(), "")
		if len(input)%2 != 0 {
			fmt.Println("NA")
			continue
		}
		ret := make([]string, 0)
		for i := 0; i < len(input); i += 2 {
			row, _ := strconv.Atoi(input[i])
			col, _ := strconv.Atoi(input[i+1])
			ret = append(ret, table[row-1][col-1])
		}
		fmt.Println(strings.Join(ret, ""))
	}
}

C++: #include <vector>
#include <list>
#include <map>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <deque>
#include <stack>
#include <bitset>
#include <algorithm>
#include <functional>
#include <numeric>
#include <utility>
#include <sstream>
#include <queue>
#include <iostream>
#include <iomanip>
#include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cctype>
#include <string>
#include <cstring>
#include <ctime>
#include <fstream>
#include <cstdio>
#include <climits>
#include <complex>
#include <cstdint>
#include <tuple>

#define M_PI       3.14159265358979323846

using namespace std;

//conversion
//------------------------------------------
inline int toInt(string s) { int v; istringstream sin(s); sin >> v; return v; }
template<class T> inline string toString(T x) { ostringstream sout; sout << x; return sout.str(); }
inline int readInt() { int x; scanf("%d", &x); return x; }

//typedef
//------------------------------------------
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<int, PII> TIII;
typedef long long LL;
typedef unsigned long long ULL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;

//container util

//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define SZ(a) int((a).size())
#define SQ(a) ((a)*(a))
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,s,n) for(int i=s;i<(int)n;++i)
#define REP(i,n) FOR(i,0,n)
#define MOD 1000000007

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define all(x) x.begin(), x.end()
#define sz(x) (int)(x).size()
struct Edge {
    int to, cost;
    Edge(int to, int cost): to(to), cost(cost) {}
};

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vector<Edge>> AdjList;
AdjList graph;

const int INF = 100000000;

int main() {
    //cout << fixed << setprecision(15);

    map<pair<int, int>, char> mp;

    mp[{1, 1}] = 'a';
    mp[{1, 2}] = 'b';
    mp[{1, 3}] = 'c';
    mp[{1, 4}] = 'd';
    mp[{1, 5}] = 'e';
    mp[{2, 1}] = 'f';
    mp[{2, 2}] = 'g';
    mp[{2, 3}] = 'h';
    mp[{2, 4}] = 'i';
    mp[{2, 5}] = 'j';
    mp[{3, 1}] = 'k';
    mp[{3, 2}] = 'l';
    mp[{3, 3}] = 'm';
    mp[{3, 4}] = 'n';
    mp[{3, 5}] = 'o';
    mp[{4, 1}] = 'p';
    mp[{4, 2}] = 'q';
    mp[{4, 3}] = 'r';
    mp[{4, 4}] = 's';
    mp[{4, 5}] = 't';
    mp[{5, 1}] = 'u';
    mp[{5, 2}] = 'v';
    mp[{5, 3}] = 'w';
    mp[{5, 4}] = 'x';
    mp[{5, 5}] = 'y';
    mp[{6, 1}] = 'z';
    mp[{6, 2}] = '.';
    mp[{6, 3}] = '?';
    mp[{6, 4}] = '!';
    mp[{6, 5}] = ' ';


    string s;
    while(cin >> s){
        if((int)s.size()%2==1) {
            cout << "NA" << endl;
            continue;
        }
        bool f = false;
        for(int i=0; i<s.size(); i+=2){
            if(mp.find({s[i]-'0', s[i+1]-'0'}) == mp.end()) f = true;
        }
        if(f){
            cout << "NA" << endl;
            continue;
        }
        for(int i=0; i<s.size(); i+=2){
            
            cout << mp[{s[i]-'0', s[i+1]-'0'}];
        }
        cout << endl;
    }
    return 0;
}

C++: #include <string>
#include <iostream>
using namespace std;


int main(){
    char map[6][5] = {
	{'a', 'b', 'c', 'd', 'e'},
	{'f', 'g', 'h', 'i', 'j'},
	{'k', 'l', 'm', 'n', 'o'},
	{'p', 'q', 'r', 's', 't'},
	{'u', 'v', 'w', 'x', 'y'},
	{'z', '.', '?', '!', ' '}};

    string s;
    while (cin >> s){
	bool err = false;
	for (int i = 0; i < s.size(); i++){
	    if (i % 2 && (s[i] > '5' || s[i] < '1')) err = true;
	    else if (i % 2 == 0 && (s[i] > '6' || s[i] < '1')) err = true;
	}
	if (s.size() % 2 || err) cout << "NA" << endl;

	else {
	    for (int i = 0; i < s.size(); i+=2){
		cout << map[s[i]-'0'-1][s[i+1]-'0'-1];
	    }
	    cout << endl;
	}
    }
}

C++: // clang-format off
#include <bits/stdc++.h>
#define int long long
#define main signed main()
// #define main int main()
#define loop(i, a, n) for (int i = (a); i < (n); i++)
#define rep(i, n) loop(i, 0, n)
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define prec(n) fixed << setprecision(n)
#define stlice(from, to) substr(from, (to) - (from) + 1)
#define pb push_back
#define mp make_pair
#define mt make_tuple
#define fi first
#define se second
using namespace std;
using pii = pair<int, int>;
using vi = vector<int>;
using vd = vector<double>;
using vc = vector<char>;
using vb = vector<bool>;
using vs = vector<string>;
using vpii = vector<pii>;
using vvi = vector<vi>;
using vvb = vector<vb>;
using vvpii = vector<vpii>;
template<typename A> using fn = function<A>;
constexpr int INF = sizeof(int) == sizeof(long long) ? 1000000000000000000LL : 1000000000;
constexpr int MOD = 1000000007;
constexpr double PI = acos(-1);
template<typename A, typename B> bool cmin(A &a, const B &b) { return a > b ? (a = b, true) : false; }
template<typename A, typename B> bool cmax(A &a, const B &b) { return a < b ? (a = b, true) : false; }
constexpr bool odd(const int &n) { return n & 1; }
constexpr bool even(const int &n) { return !odd(n); }
void solve();
main { solve(); return 0; }
// clang-format on

void solve() {
  string t[] = {"abcde", "fghij", "klmno", "pqrst", "uvwxy", "z.?! "};
  string s;
  while (cin >> s) {
    if (odd(s.size())) {
      cout << "NA" << endl;
      continue;
    }
    stringstream ss;
    for (int i = 0; i < s.size(); i += 2) {
      if (s[i] < '1' || '6' < s[i] || s[i + 1] < '1' || '5' < s[i + 1]) {
        cout << "NA" << endl;
        goto next;
      }
      ss << t[s[i] - '1'][s[i + 1] - '1'];
    }
    cout << ss.str() << endl;
  next:;
  }
}

C++: #include <iostream>
#include <string>
using namespace std;

string s, r;

inline int cast(char c) {
	return (int)(c - '0');
}

int main(){
	bool fg;
	int a, b;
	while (cin >> s) {
		fg = false;
		r = "";
		if (s.size() % 2 != 0) {
			cout << "NA" << endl;
			continue;
		}
		for (int i = 0; i < s.size(); i += 2) {
			a = cast(s[i]), b = cast(s[i + 1]);
			if (a > 6 || b > 5 || a < 1 || b < 1) {
				fg = true;
				break;
			}
			if (a < 6) {
				r += ('a' + ((a - 1) * 5 + (b - 1)));
			}
			else {
				switch (b) {
				case 1:r += 'z';
					break;
				case 2:r += '.';
					break;
				case 3:r += '?';
					break;
				case 4:r += '!';
					break;
				case 5:r += ' ';
					break;
				default:break;
				}
			}
		}
		if (fg) {
			cout << "NA" << endl;
		}
		else {
			cout << r << endl;
		}
	}

	return 0;
}