#pragma GCC optimize ("Ofast")
#include<bits/stdc++.h>
using namespace std;
#define MD (998244353U)
struct modint{
  static unsigned md;
  unsigned val;
  modint(){
    val=0;
  }
  modint(int a){
    val = ord(a);
  }
  modint(unsigned a){
    val = ord(a);
  }
  modint(long long a){
    val = ord(a);
  }
  modint(unsigned long long a){
    val = ord(a);
  }
  void setmod(unsigned m){
    md = m;
  }
  unsigned ord(unsigned a){
    return a%md;
  }
  unsigned ord(int a){
    a %= (int)md;
    if(a < 0){
      a += md;
    }
    return a;
  }
  unsigned ord(unsigned long long a){
    return a%md;
  }
  unsigned ord(long long a){
    a %= (int)md;
    if(a < 0){
      a += md;
    }
    return a;
  }
  unsigned get(){
    return val;
  }
  inline modint &operator++(){
    val++;
    if(val >= md){
      val -= md;
    }
    return *this;
  }
  inline modint &operator--(){
    if(val == 0){
      val = md - 1;
    }
    else{
      --val;
    }
    return *this;
  }
  inline modint operator++(int a){
    modint res(*this);
    val++;
    if(val >= md){
      val -= md;
    }
    return res;
  }
  inline modint operator--(int a){
    modint res(*this);
    if(val == 0){
      val = md - 1;
    }
    else{
      --val;
    }
    return res;
  }
  modint &operator+=(modint a){
    val += a.val;
    if(val >= md){
      val -= md;
    }
    return *this;
  }
  modint &operator-=(modint a){
    if(val < a.val){
      val = val + md - a.val;
    }
    else{
      val -= a.val;
    }
    return *this;
  }
  modint &operator*=(modint a){
    val = ((unsigned long long)val*a.val)%md;
    return *this;
  }
  modint &operator/=(modint a){
    return *this *= a.inverse();
  }
  modint operator+(modint a){
    return modint(*this)+=a;
  }
  modint operator-(modint a){
    return modint(*this)-=a;
  }
  modint operator*(modint a){
    return modint(*this)*=a;
  }
  modint operator/(modint a){
    return modint(*this)/=a;
  }
  modint operator+(int a){
    return modint(*this)+=modint(a);
  }
  modint operator-(int a){
    return modint(*this)-=modint(a);
  }
  modint operator*(int a){
    return modint(*this)*=modint(a);
  }
  modint operator/(int a){
    return modint(*this)/=modint(a);
  }
  modint operator+(long long a){
    return modint(*this)+=modint(a);
  }
  modint operator-(long long a){
    return modint(*this)-=modint(a);
  }
  modint operator*(long long a){
    return modint(*this)*=modint(a);
  }
  modint operator/(long long a){
    return modint(*this)/=modint(a);
  }
  modint operator-(void){
    modint res;
    if(val){
      res.val=md-val;
    }
    else{
      res.val=0;
    }
    return res;
  }
  operator bool(void){
    return val!=0;
  }
  operator int(void){
    return get();
  }
  operator long long(void){
    return get();
  }
  modint inverse(){
    int a = val;
    int b = md;
    int u = 1;
    int v = 0;
    int t;
    modint res;
    while(b){
      t = a / b;
      a -= t * b;
      swap(a, b);
      u -= t * v;
      swap(u, v);
    }
    if(u < 0){
      u += md;
    }
    res.val = u;
    return res;
  }
  modint pw(unsigned long long b){
    modint a(*this);
    modint res;
    res.val = 1;
    while(b){
      if(b&1){
        res *= a;
      }
      b >>= 1;
      a *= a;
    }
    return res;
  }
  bool operator==(int a){
    return ord(a)==val;
  }
  bool operator!=(int a){
    return ord(a)!=val;
  }
}
;
unsigned modint::md;
modint operator+(int a, modint b){
  return modint(a)+=b;
}
modint operator-(int a, modint b){
  return modint(a)-=b;
}
modint operator*(int a, modint b){
  return modint(a)*=b;
}
modint operator/(int a, modint b){
  return modint(a)/=b;
}
modint operator+(long long a, modint b){
  return modint(a)+=b;
}
modint operator-(long long a, modint b){
  return modint(a)-=b;
}
modint operator*(long long a, modint b){
  return modint(a)*=b;
}
modint operator/(long long a, modint b){
  return modint(a)/=b;
}
inline int my_getchar(){
  static char buf[1048576];
  static int s = 1048576;
  static int e = 1048576;
  if(s == e && e == 1048576){
    e = fread(buf, 1, 1048576, stdin);
    s = 0;
  }
  if(s == e){
    return EOF;
  }
  return buf[s++];
}
inline void rd(int &x){
  int k;
  int m=0;
  x=0;
  for(;;){
    k = my_getchar();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = my_getchar();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
inline void rd(char &c){
  int i;
  for(;;){
    i = my_getchar();
    if(i!=' '&&i!='\n'&&i!='\r'&&i!='\t'&&i!=EOF){
      break;
    }
  }
  c = i;
}
inline int rd(char c[]){
  int i;
  int sz = 0;
  for(;;){
    i = my_getchar();
    if(i!=' '&&i!='\n'&&i!='\r'&&i!='\t'&&i!=EOF){
      break;
    }
  }
  c[sz++] = i;
  for(;;){
    i = my_getchar();
    if(i==' '||i=='\n'||i=='\r'||i=='\t'||i==EOF){
      break;
    }
    c[sz++] = i;
  }
  c[sz]='\0';
  return sz;
}
struct MY_WRITER{
  char buf[1048576];
  int s;
  int e;
  MY_WRITER(){
    s = 0;
    e = 1048576;
  }
  ~MY_WRITER(){
    if(s){
      fwrite(buf, 1, s, stdout);
    }
  }
}
;
MY_WRITER MY_WRITER_VAR;
void my_putchar(int a){
  if(MY_WRITER_VAR.s == MY_WRITER_VAR.e){
    fwrite(MY_WRITER_VAR.buf, 1, MY_WRITER_VAR.s, stdout);
    MY_WRITER_VAR.s = 0;
  }
  MY_WRITER_VAR.buf[MY_WRITER_VAR.s++] = a;
}
inline void wt_L(char a){
  my_putchar(a);
}
inline void wt_L(int x){
  int s=0;
  int m=0;
  char f[10];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar('-');
  }
  while(s--){
    my_putchar(f[s]+'0');
  }
}
inline void wt_L(modint x){
  int i;
  i = (int)x;
  wt_L(i);
}
int N;
char S[50000+10];
int s[50000+10];
modint dp[28][28];
modint ndp[28][28];
int main(){
  int i, j;
  {
    modint x;
    x.setmod(MD);
  }
  rd(N);
  rd(S);
  if(N == 1){
    int res = 0 ;
    if(S[0] == '?'){
      res = int('z' - 'a' + 1);
    }
    else{
      res =  1;
    }
    wt_L(res);
    wt_L('\n');
    exit(0);
  }
  for(i=(0);i<(N);i++){
    if('a' <= S[i]  &&  S[i] <= 'z'){
      s[i] = S[i] - 'a';
    }
    else{
      s[i] = -1;
    }
  }
  for(i=('a');i<('z'+1);i++){
    int j;
    if(S[0] != '?' && S[0] != i){
      continue;
    }
    for(j=('a');j<('z' + 1);j++){
      if(S[1] != '?' && S[1] != j){
        continue;
      }
      if(i == j){
        continue;
      }
      ++dp[i-'a'][j-'a'];
    }
  }
  for(i=(2);i<(N);i++){
    int j;
    for(j=('a');j<('z' + 1);j++){
      int k;
      for(k=('a');k<('z' + 1);k++){
        ndp[j-'a'][k-'a'] = 0;
      }
    }
    for(j=('a');j<('z' + 1);j++){
      int k;
      if(S[i] != '?' && S[i] != j){
        continue;
      }
      for(k=('a');k<('z' + 1);k++){
        int lk;
        if(k == j){
          continue;
        }
        for(lk=('a');lk<('z'+1);lk++){
          if(lk == j){
            continue;
          }
          if(lk == k){
            continue;
          }
          ndp[k-'a'][j-'a'] += dp[lk-'a'][k-'a'];
        }
      }
    }
    for(j=('a');j<('z' + 1);j++){
      int k;
      for(k=('a');k<('z' + 1);k++){
        dp[j-'a'][k-'a'] = ndp[j-'a'][k-'a'];
      }
    }
  }
  modint res;
  res = 0;
  for(j=('a');j<('z' + 1);j++){
    int k;
    for(k=('a');k<('z' + 1);k++){
      res += dp[j-'a'][k-'a'];
    }
  }
  wt_L(res);
  wt_L('\n');
  return 0;
}
// cLay version 20210405-1

// --- original code ---
// //no-unlocked
// #define MD 998244353
// int N;
// char S[5d4+10];
// int s[5d4+10];
// modint dp[28][28];
// modint ndp[28][28];
// { 
//     rd(N, S);
//     if(N == 1){
//         int res = 0 ;
//         if(S[0] == '?') res = int('z' - 'a' + 1);
//         else res =  1;
//         wt(res);
//         exit(0);
//     }
//     rep(i, N){
//         if('a' <= S[i] <= 'z') s[i] = S[i] - 'a';
//         else s[i] = -1;
//     }
//     rep(i, 'a', 'z'+1){
//         if(S[0] != '?' && S[0] != i) continue;
//         rep(j, 'a', 'z' + 1){
//             if(S[1] != '?' && S[1] != j) continue;
//             if(i == j) continue;
//             ++dp[i-'a'][j-'a'];
//         }
//     }
//     rep(i, 2, N){
//         rep(j, 'a', 'z' + 1){
//             rep(k, 'a', 'z' + 1) ndp[j-'a'][k-'a'] = 0;
//         }
//         rep(j, 'a', 'z' + 1){
//             if(S[i] != '?' && S[i] != j) continue;
//             rep(k, 'a', 'z' + 1){
//                 if(k == j) continue;
//                 rep(lk, 'a', 'z'+1){
//                     if(lk == j) continue;
//                     if(lk == k) continue;
//                     ndp[k-'a'][j-'a'] += dp[lk-'a'][k-'a'];
//                 }
//             }
//         }
//         rep(j, 'a', 'z' + 1){
//             rep(k, 'a', 'z' + 1)  dp[j-'a'][k-'a'] = ndp[j-'a'][k-'a'];
//         }
//     }
//     modint res;
//     res = 0;
//     rep(j, 'a', 'z' + 1){
//         rep(k, 'a', 'z' + 1)  res += dp[j-'a'][k-'a'];
//     }
//     wt(res);
// }