B. Лиса и имена

#include <bits/stdc++.h>

using i32 = int;
using ui32 = unsigned int;
using i64 = long long;
using ui64 = unsigned long long;


const ui32 MSIZE = 26;

bool g[MSIZE][MSIZE];
std::string arr[100];
ui32 color[MSIZE];

bool CycleExists(ui32 from) {
  color[from] = 1;
  for (ui32 i = 0; i < MSIZE; i++) {
    if (g[from][i]) {
      if (color[i] == 1) return true;
      if (CycleExists(i)) return true;
    }
  }
  color[from] = 2;
  return false;
}

void TopsortDfs(ui32 from, std::vector<ui32>& result) {
  color[from] = true;
  for (ui32 i = 0; i < MSIZE; i++) {
    if (g[from][i] == true and color[i] == false) {
      TopsortDfs(i, result);
    }
  }
  result.push_back(from);
}

i32 main() {
  ui32 n;
  std::cin >> n;
  for (ui32 i = 0; i < n; i++) {
    std::cin >> arr[i];
    for (ui32 j = 0; j < std::size(arr[i]); j++) {
      arr[i][j] -= 'a';
    }
  }
  bool alphabetExists = true;
  for (ui32 i = 0; i + 1 < n; i++) {
    ui32 minLength = std::min(std::size(arr[i]), std::size(arr[i + 1]));
    bool cmp = false;
    for (ui32 j = 0; j < minLength; j++) {
      if (arr[i][j] != arr[i + 1][j]) {
        g[arr[i][j]][arr[i + 1][j]] = true;
        cmp = true;
        break;
      }
    }
    if (cmp == false and std::size(arr[i]) > std::size(arr[i + 1])) {
      alphabetExists = false;
      break;
    }
  }
  for (ui32 i = 0; i < MSIZE; i++) {
    if (color[i] == 0) {
      if (CycleExists(i)) {
        alphabetExists = false;
        break;
      }
    }
  }
  if (alphabetExists == false) {
    puts("Impossible");
    return 0;
  }
  std::fill(color, color + MSIZE, 0); // equals to used
  std::vector<ui32> alphabet;
  for (ui32 i = 0; i < MSIZE; i++) {
    if (color[i] == false) {
      TopsortDfs(i, alphabet);
    }
  }
  std::reverse(std::begin(alphabet), std::end(alphabet));
  std::string answer(26, 'a');
  for (ui32 i = 0; i < MSIZE; i++) {
    answer[i] += alphabet[i];
  }
  puts(answer.c_str());
}