func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     string s;     int i, j, n, k, imp, c, h;     int f[200];     {       imp = 0;       memset(f, 0, sizeof(f));       cin >> k;       cin >> s;       n = ((int)s.size());       h = n % 2 + n / 2;       for (i = 0; i < n; i++)         if (isalpha(s[i])) f[s[i] -  a ]++;       for (i = h - 1; i > -1; i--) {         if (s[i] != s[n - i - 1]) {           if (isalpha(s[i]) && isalpha(s[n - i - 1])) {             imp = 1;             break;           } else if (isalpha(s[i]))             s[n - i - 1] = s[i];           else             s[i] = s[n - i - 1];         } else if (s[i] ==  ? ) {           c = 0;           for (j = k - 1; j > -1; j--)             if (f[j] == 0) {               c = j;               f[j]++;               break;             }           s[i] = s[n - i - 1] =  a  + c;         }       }       if (imp == 0) {         for (i = 0; i < k; i++)           if (!f[i]) {             imp = 1;             break;           }       }       if (!imp)         cout << s << endl;       else         cout <<  IMPOSSIBLE  << endl;     }     return 0;   }   