Problem:
Score: 600 points

Problem Statement

How many strings can be obtained by applying the following operation on a string S exactly K times: "choose one lowercase English letter and insert it somewhere"?

The answer can be enormous, so print it modulo (10^9+7).

Constraints

K is an integer between 1 and 10^6 (inclusive).

S is a string of length between 1 and 10^6 (inclusive) consisting of lowercase English letters.

Input

Input is given from Standard Input in the following format:

K
S

Output

Print the number of strings satisfying the condition, modulo (10^9+7).

Sample Input 1

5
oof

Sample Output 1

575111451

For example, we can obtain proofend, moonwolf, and onionpuf, while we cannot obtain oofsix, oofelevennn, voxafolt, or fooooooo.

Sample Input 2

37564
whydidyoudesertme

Sample Output 2

318008117

Solution in Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.io.*;
import java.math.BigDecimal;
import java.util.*;

@SuppressWarnings("unused")
public class Main {
    FastScanner in;
    PrintWriter out;
    final static int MOD = (int)1e9+7;
    
    void solve(){
        int K = in.nextInt();
        String S = in.nextStr();
        
        int L = S.length();
        ModCalculator mc = new ModCalculator(K + L, MOD);
        long ans = 0;
        for(int i = L - 1; i < K + L; i++){
            long add = 1;
            add *= mc.modComb(i, L - 1);
            add %= MOD;
            add *= ModCalculator.modPow(25, i - (L - 1), MOD);
            add %= MOD;
            add *= ModCalculator.modPow(26, (K + L - 1) - i, MOD);
            add %= MOD;
            ans += add;
            ans %= MOD;
        }
        out.println(ans);
    }
    
    static class ModCalculator{
        //引数の最大値、MOD
        private final int MAX_FAC, MOD;
        //階乗・階乗の逆元・積の逆元のテーブル
        private long[] fact, factInv, inv;
        
        public ModCalculator(){
            this(510000, (int)1e9+7);
        }
        
        public ModCalculator(int max, int mod){
            MAX_FAC = max; MOD = mod;
            init();
        }
        
        //テーブルの初期化
        private void init(){
            fact = new long[MAX_FAC+1];
            factInv = new long[MAX_FAC+1];
            inv = new long[MAX_FAC+1];
            fact[0] = factInv[0] = inv[0] = fact[1] = factInv[1] = inv[1] = 1;
            for (int i = 2; i <= MAX_FAC; ++i) {
                fact[i] = fact[i - 1] * i % MOD;
                inv[i] = MOD - (MOD / i) * inv[(int) (MOD % i)] % MOD;
                factInv[i] = factInv[i - 1] * inv[i] % MOD;
            }
        }
        
        //nPk % MOD
        public long modPerm(int n, int k){
            return (n >= k) ? fact[n] * factInv[n - k] % MOD : 0;
        }
        
        //nCk % MOD
        public long modComb(int n, int k){
            return (n >= k) ? fact[n] * factInv[k] % MOD * factInv[n - k] % MOD : 0;
        }
        
        //nPk % MOD、nが大きくてもO(k)で計算できる
        public long modPermLarge(long n, int k){
            if(n < k) return 0;
            long res = 1;
            for(long i = n; i > n - k; i--) res = (res * (i % MOD)) % MOD;
            return res;
        }
        
        //nCk % MOD、nが大きくてもO(k)で計算できる
        public long modCombLarge(long n, int k){
            if(n < k) return 0;
            long res = 1;
            for(long i = n; i > n - k; i--) res = (res * (i % MOD)) % MOD;
            res = (res * factInv[k]) % MOD;
            return res;
        }
        
        //n! % MOD
        public long modFact(int n){
            return fact[n];
        }
        
        //(n!)^-1 % MOD
        public long modFactInv(int n){
            return factInv[n];
        }
        
        //n^-1 % MOD
        public long modMultiInv(int n){
            return inv[n];
        }
        
        //n^-1 % MOD をlog(MOD)で計算
        public static long modMultiInv(long a, int mod){
            return modPow(a, mod-2, mod);
        }
        
        //a^b % MOD をlog(b)で計算
        public static long modPow(long a, long b, int mod){
            long res = 1;
            while(b > 0){
                if((b & 1) == 1) res = (res * a) % mod;
                a = (a * a) % mod;
                b = b >> 1;
            }
            return res;
        }
        
        public static long[] buildModPow(long a, int maxPow, int mod){
            long[] pow = new long[maxPow+1];
            pow[0] = 1;
            for(int i = 1; i <= maxPow; i++) pow[i] = (pow[i-1] * (a % mod)) % mod;
            return pow;
        }
    }
    
    public static void main(String[] args) {
        new Main().m();
    }
    
    private void m() {
        in = new FastScanner(System.in);
        out = new PrintWriter(System.out);
        /*
        try {
            String path = "input.txt";
            out = new PrintWriter(new BufferedWriter(new FileWriter(new File(path))));
        }catch (IOException e){
            e.printStackTrace();
        }
        */
        solve();
        out.flush();
        in.close();
        out.close();
    }
    
    static class FastScanner {
        private Reader input;
        
        public FastScanner() {this(System.in);}
        public FastScanner(InputStream stream) {this.input = new BufferedReader(new InputStreamReader(stream));}
        public void close() {
            try {
                this.input.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        
        public int nextInt() {
            long nl = nextLong();
            if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException();
            return (int) nl;
        }
        
        public long nextLong() {
            try {
                int sign = 1;
                int b = input.read();
                while ((b < '0' || '9' < b) && b != '-' && b != '+') {
                    b = input.read();
                }
                if (b == '-') {
                    sign = -1;
                    b = input.read();
                } else if (b == '+') {
                    b = input.read();
                }
                long ret = b - '0';
                while (true) {
                    b = input.read();
                    if (b < '0' || '9' < b) return ret * sign;
                    ret *= 10;
                    ret += b - '0';
                }
            } catch (IOException e) {
                e.printStackTrace();
                return -1;
            }
        }
        
        public double nextDouble() {
            try {
                double sign = 1;
                int b = input.read();
                while ((b < '0' || '9' < b) && b != '-' && b != '+') {
                    b = input.read();
                }
                if (b == '-') {
                    sign = -1;
                    b = input.read();
                } else if (b == '+') {
                    b = input.read();
                }
                double ret = b - '0';
                while (true) {
                    b = input.read();
                    if (b < '0' || '9' < b) break;
                    ret *= 10;
                    ret += b - '0';
                }
                if (b != '.') return sign * ret;
                double div = 1;
                b = input.read();
                while ('0' <= b && b <= '9') {
                    ret *= 10;
                    ret += b - '0';
                    div *= 10;
                    b = input.read();
                }
                return sign * ret / div;
            } catch (IOException e) {
                e.printStackTrace();
                return Double.NaN;
            }
        }
        
        public char nextChar() {
            try {
                int b = input.read();
                while (Character.isWhitespace(b)) {
                    b = input.read();
                }
                return (char) b;
            } catch (IOException e) {
                e.printStackTrace();
                return 0;
            }
        }
        
        public String nextStr() {
            try {
                StringBuilder sb = new StringBuilder();
                int b = input.read();
                while (Character.isWhitespace(b)) {
                    b = input.read();
                }
                while (b != -1 && !Character.isWhitespace(b)) {
                    sb.append((char) b);
                    b = input.read();
                }
                return sb.toString();
            } catch (IOException e) {
                e.printStackTrace();
                return "";
            }
        }
        
        public String nextLine() {
            try {
                StringBuilder sb = new StringBuilder();
                int b = input.read();
                while (b != -1 && b != '\n') {
                    sb.append((char) b);
                    b = input.read();
                }
                return sb.toString();
            } catch (IOException e) {
                e.printStackTrace();
                return "";
            }
        }
        
        public int[] nextIntArray(int n) {
            int[] res = new int[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextInt();
            }
            return res;
        }
        
        public int[] nextIntArrayDec(int n) {
            int[] res = new int[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextInt() - 1;
            }
            return res;
        }
        
        public int[] nextIntArray1Index(int n) {
            int[] res = new int[n + 1];
            for (int i = 0; i < n; i++) {
                res[i + 1] = nextInt();
            }
            return res;
        }
        
        public long[] nextLongArray(int n) {
            long[] res = new long[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextLong();
            }
            return res;
        }
        
        public long[] nextLongArrayDec(int n) {
            long[] res = new long[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextLong() - 1;
            }
            return res;
        }
        
        public long[] nextLongArray1Index(int n) {
            long[] res = new long[n + 1];
            for (int i = 0; i < n; i++) {
                res[i + 1] = nextLong();
            }
            return res;
        }
        
        public double[] nextDoubleArray(int n) {
            double[] res = new double[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextDouble();
            }
            return res;
        }
    }
}