#include <bits/stdc++.h>   using namespace std;   const double eps = 1e-6;   void __print(int x) { cerr << x; }   void __print(long x) { cerr << x; }   void __print(long long x) { cerr << x; }   void __print(unsigned x) { cerr << x; }   void __print(unsigned long x) { cerr << x; }   void __print(unsigned long long x) { cerr << x; }   void __print(float x) { cerr << x; }   void __print(double x) { cerr << x; }   void __print(long double x) { cerr << x; }   void __print(char x) { cerr <<      << x <<     ; }   void __print(const char *x) { cerr <<     << x <<    ; }   void __print(const string &x) { cerr <<     << x <<    ; }   void __print(bool x) { cerr << (x ?  true  :  false ); }   template <typename T, typename V>   void __print(const pair<T, V> &x) {     cerr <<  { ;     __print(x.first);     cerr <<  , ;     __print(x.second);     cerr <<  } ;   }   template <typename T>   void __print(const T &x) {     int f = 0;     cerr <<  { ;     for (auto &i : x) cerr << (f++ ?  ,  :   ), __print(i);     cerr <<  } ;   }   void _print() { cerr <<  ] n ; }   template <typename T, typename... V>   void _print(T t, V... v) {     __print(t);     if (sizeof...(v)) cerr <<  ,  ;     _print(v...);   }   const long long N = 2010;   long long n;   long long k;   string s[N];   string ans;   long long dp[N][N];   long long path[N][N];   void solve() {     cin >> n;     cin >> k;     for (long long i = 0; i < n; ++i) cin >> s[i];     for (long long i = 0; i < n; ++i) {       for (long long j = 0; j < n; ++j) {         if (i == 0 && j == 0) {           dp[i][j] += (s[i][j] !=  a );           continue;         }         if (i == 0) {           dp[i][j] = dp[i][j - 1];           dp[i][j] += (s[i][j] !=  a );           continue;         }         if (j == 0) {           dp[i][j] = dp[i - 1][j];           dp[i][j] += (s[i][j] !=  a );           continue;         }         dp[i][j] += min(dp[i - 1][j], dp[i][j - 1]) + (s[i][j] !=  a );       }     }     if (k >= 2 * n - 1) {       ans = string(2 * n - 1,  a );       cout << ans <<   n ;       return;     }     set<pair<long long, long long>> pool;     long long len = -1;     for (long long i = 0; i < n; ++i) {       for (long long j = 0; j < n; ++j) {         if (dp[i][j] <= k) {           len = max(len, i + j);         }       }     }     if (len == -1) {       ans.push_back(s[0][0]);       len = 0;       if (1 < n) {         pool.insert({0, 1});         pool.insert({1, 0});       }     } else {       ans = string(len + 1,  a );       for (long long i = 0; i < n; ++i) {         for (long long j = 0; j < n; ++j) {           if (i + j == len && dp[i][j] <= k) {             if (i + 1 < n) pool.insert({i + 1, j});             if (j + 1 < n) pool.insert({i, j + 1});           }         }       }     }     long long req = 2 * n - 1 - len - 1;     ;     while (req > 0) {       char c =  z ;       for (auto it : pool) {         c = min(c, s[it.first][it.second]);       }       vector<pair<long long, long long>> toerase;       for (auto it : pool) {         if (c != s[it.first][it.second]) toerase.push_back(it);       }       for (auto it : toerase) pool.erase(it);       vector<pair<long long, long long>> topush;       for (auto [i, j] : pool) {         if (i + 1 < n) topush.push_back({i + 1, j});         if (j + 1 < n) topush.push_back({i, j + 1});       }       pool.clear();       for (auto it : topush) pool.insert(it);       req--;       ans.push_back(c);     }     cout << ans <<   n ;   }   signed main() {     ios::sync_with_stdio(false);     cin.tie(NULL);     ;     long long t = 1;     while (t--) solve();   }