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])
			// log.Println(table[row-1][col-1])
			ret = append(ret, table[row-1][col-1])
		}
		fmt.Println(strings.Join(ret, ""))
	}
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <cstring>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <stack>
#include <queue>

using namespace std;
typedef long long LL;
typedef pair<int, int> PII;
static const double EPS = 1e-5;

#define FOR(i,k,n) for (int i=(k); i<(int)(n); ++i)
#define REP(i,n) FOR(i,0,n)
string a = "afkpu";
int main(void){
  string line;
  while(cin>>line){
    string ans;
    bool na = false;
    if(line.size()%2){
      na = true;
    }
    for(int i=0;i<(int)line.size(); i+=2){
      if(na) break;
      if(line[i+1]>='6'||line[i+1]<='0'){
        na = true;
        break;
      }
      if(line[i]<='5'){
        ans+=(char)(a[line[i]-'1']+line[i+1]-'1');
      }else if(line[i]=='6'){
        if(line[i+1]=='1') ans+='z';
        if(line[i+1]=='2') ans+='.';
        if(line[i+1]=='3') ans+='?';
        if(line[i+1]=='4') ans+='!';
        if(line[i+1]=='5') ans+=' ';
      }else{
        na = true;
        break;
      }
    }
    if(!na)cout<<ans<<endl;
    else cout<<"NA"<<endl;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int main(){
  string str[6]={"afkpuz","bglqv.","chmrw?"
		 ,"dinsx!","ejoty "};
  string str2;
  while(cin>>str2){
    if(str2.size()%2==1)
      cout<<"NA"<<endl;
    else{
      string ans="";
      for(int i=0;i<str2.size();i+=2){
	if(str2[i+1]-'0'<=5&&str2[i]-'0'<=6&&str2[i+1]-'0'>0&&str2[i]-'0'>0){
	ans+=str[str2[i+1]-'0'-1][str2[i]-'0'-1];
	}
	else{
	  ans="NA";
	}
      }
      cout<<ans<<endl;
    }
  }
  return 0;
}

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

int index[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', '.', '?', '!', ' '},
};

int main() {
  string s;
  while(cin >> s) {
    if(s.size()%2) cout << "NA" << endl;
    else {
      string ans;
      for(int i = 0; i < s.size(); i += 2) {
        int column = s[i]-'1', row = s[i+1]-'1';
        if(column > 5 || row > 4) {
          cout << "NA" << endl;
          break;
        }
        ans += index[column][row];
        if(i+2 == s.size()) cout << ans << endl;
      }
    }
  }
}

C++: #include<iostream>
#include<string>
using namespace std;
int main(){
  string str;
  char hyou[6][7];
  hyou[1][6] = 'z';
  hyou[2][6] = '.';
  hyou[3][6] = '?';
  hyou[4][6] = '!';
  hyou[5][6] = ' ';
  for(int i=1;i<=5;i++){
    for(int j=1;j<=5;j++){
      hyou[i][j] = 'a'+(j-1)*5+i-1;
    }
  }
  
  while(cin >> str){
    if(str.size()%2 != 0){
      cout << "NA" << endl;
      continue;
    }
    for(int i=0;i<str.size();i+=2){
      cout << hyou[ str[i+1]-'0'][str[i]-'0'];
    } cout << endl;
  }

  return (0);
}

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;
	}
    }
}