import java.util.*;
import java.io.*;

public class Main {
	static final int INF = 1<<30;
	static final long INFL = 1L<<60;
	static final int MOD = 1000000007;
	static final int MOD2 = 998244353;
	static List<List<Integer>> g;
	
	public static void main(String[] args) {
		FastScanner fs = new FastScanner();
		PrintWriter pw = new PrintWriter(System.out);
		// dp[i][j][k] := i文字目までみてi文字目jでi-1文字目がkであるとき
		// a b !(b,a)
		int n = fs.nextInt();
		char[] s = fs.nextChars();
		long[][][] dp = new long[n][27][27];	
		for(int i = 0; i < n; i++){
			if(i == 0){
				if(s[i] == '?'){
					for(int j = 1; j <= 26; j++){
						dp[i][j][0] = 1;
					}
				} else {
					int c = s[i]-'a'+1;
					dp[i][c][0] = 1;
				}
			} else if(i == 1){
				if(s[i] == '?'){
					for(int j = 1; j <= 26; j++){
						for(int k = 1; k <= 26; k++){
							if(j != k){
								dp[i][j][k] += dp[i-1][k][0];
							}
						}
					}
				} else {
					int c = s[i]-'a'+1;
					for(int k = 1; k <= 26; k++){
						if(c != k){
							dp[i][c][k] += dp[i-1][k][0];
						}
					}
				}
			} else {
				if(s[i] == '?'){
					// a??b
					long[] acm = new long[27];
					for(int k = 1; k <= 26; k++){
						for(int l = 1; l <= 26; l++){
							acm[k] += dp[i-1][k][l];
							acm[k] %= MOD2;
						}
					}
					for(int j = 1; j <= 26; j++){
						for(int k = 1; k <= 26; k++){
							if(j != k){
								dp[i][j][k] += acm[k] - dp[i-1][k][j];
								dp[i][j][k] = (dp[i][j][k] + MOD2) % MOD2;
							}
						}
					}
				} else {
					int c = s[i]-'a'+1;
					for(int k = 1; k <= 26; k++){
						for(int l = 1; l <= 26; l++){
							if(c != k && c != l){
								dp[i][c][k] += dp[i-1][k][l];
								dp[i][c][k] %= MOD2;
							}
						}
					}
				}
			}

		}
		long ans = 0;
		for(int j = 0; j <= 26; j++){
			for(int k = 0; k <= 26; k++){
				ans += dp[n-1][j][k];
				ans %= MOD2;
			}
		}
		pw.println(ans);
	
		pw.close();
	}
}

class FastScanner {
	private InputStream in = System.in;
	private byte[] buffer = new byte[1024];
	private int length = 0, p = 0;
	private boolean hasNextByte() {
		if (p < length) return true;
		else{
			p = 0;
			try{
				length = in.read(buffer);
			}catch(Exception e){
				e.printStackTrace();
			}
			if(length == 0) return false;
		}
		return true;
	}
	private int readByte() {
		if (hasNextByte() == true) return buffer[p++];
		return -1;
	}
	private static boolean isPrintable(int n) {
		return 33 <= n && n <= 126;
	}
	private void skip() {
		while (hasNextByte() && !isPrintable(buffer[p])) p++;
	}
	public boolean hasNext() {
		skip();
		return hasNextByte();
	}
	public String next() {
		if(!hasNext()) throw new NoSuchElementException();
		StringBuilder sb = new StringBuilder();
		int temp = readByte();
		while (isPrintable(temp)) {
			sb.appendCodePoint(temp);
			temp = readByte();
		}
		return sb.toString();
	}
	public int nextInt() {
		return Math.toIntExact(nextLong());
	}
	public int[] nextInts(int n) {
		int[] ar = new int[n];
		for (int i = 0; i < n; i++) ar[i] = nextInt();
		return ar;
	}
	public long[] nextLongs(int n) {
		long[] ar = new long[n];
		for (int i = 0; i < n; i++) ar[i] = nextLong();
		return ar;
	}
	public long nextLong() {
		if(!hasNext()) throw new NoSuchElementException();
		boolean minus = false;
		int temp = readByte();
		if (temp == '-') {
			minus = true;
			temp = readByte();
		}
		if (temp < '0' || '9' < temp) throw new NumberFormatException();
		long n = 0;
		while (isPrintable(temp)) {
			if ('0' <= temp && temp <= '9') {
				n *= 10;
				n += temp - '0';
			}
			temp = readByte();
		}
		return minus ? -n : n;
	}
	public char[] nextChars() {
		String s = next();
		return s.toCharArray();
	}
	public char[][] nextCharMat(int h, int w) {
		char[][] ar = new char[h][w];
		for(int i = 0; i < h; i++){
			String s = next();
			for(int j = 0; j < w; j++){
				ar[i][j] = s.charAt(j);
			}
		}
		return ar;
	}
}