#include <bits/stdc++.h>
using namespace std;
template <typename T, typename U>
inline void smin(T &a, const U &b) {
  if (a > b) a = b;
}
template <typename T, typename U>
inline void smax(T &a, const U &b) {
  if (a < b) a = b;
}
template <typename T>
inline void gn(T &first) {
  char c, sg = 0;
  while (c = getchar(), (c > '9' || c < '0') && c != '-')
    ;
  for ((c == '-' ? sg = 1, c = getchar() : 0), first = 0; c >= '0' && c <= '9';
       c = getchar())
    first = (first << 1) + (first << 3) + c - '0';
  if (sg) first = -first;
}
template <class T, class T1>
inline void gn(T &first, T1 &second) {
  gn(first);
  gn(second);
}
template <class T, class T1, class T2>
inline void gn(T &first, T1 &second, T2 &z) {
  gn(first);
  gn(second);
  gn(z);
}
template <typename T>
inline void print(T first) {
  if (first < 0) {
    putchar('-');
    return print(-first);
  }
  if (first < 10) {
    putchar('0' + first);
    return;
  }
  print(first / 10);
  putchar(first % 10 + '0');
}
template <typename T>
inline void println(T first) {
  print(first), putchar('\n');
}
template <typename T>
inline void printsp(T first) {
  print(first), putchar(' ');
}
template <class T, class T1>
inline void print(T first, T1 second) {
  printsp(first), println(second);
}
template <class T, class T1, class T2>
inline void print(T first, T1 second, T2 z) {
  printsp(first), printsp(second), println(z);
}
int power(int a, int b, int m, int ans = 1) {
  for (; b; b >>= 1, a = (long long)a * a % m)
    if (b & 1) ans = (long long)ans * a % m;
  return ans;
}
const int N = 2222;
char s[N][N];
int d[N][N], flag[N][N];
int main() {
  int n, m;
  gn(n, m);
  for (int i = 1; i <= n; i++) scanf("%s", s[i] + 1);
  for (int i = 1; i <= n; i++) s[i][n + 1] = s[n + 1][i] = 'z';
  d[1][1] = (s[1][1] == 'a');
  for (int r = 2; r <= n * 2; r++)
    for (int i = 1; i <= n; i++) {
      int j = r - i;
      if (j > 0 && j <= n) {
        if (i > 1) smax(d[i][j], d[i - 1][j] + ('a' == s[i][j]));
        if (j > 1) smax(d[i][j], d[i][j - 1] + ('a' == s[i][j]));
      }
    }
  int k = 1;
  for (int i = 1; i <= n; i++)
    for (int j = 1; j <= n; j++) {
      if (i + j - 1 <= d[i][j] + m) smax(k, i + j);
    }
  for (int i = 1; i < k; i++) putchar('a');
  if (k > 1) {
    for (int i = 1; i <= n; i++) {
      int j = k - i;
      if (j > 0 && j <= n && i + j - 1 <= d[i][j] + m) {
        flag[i][j] = 1;
      }
    }
  } else {
    flag[1][1] = 1;
    putchar(s[1][1]);
  }
  char t;
  for (int r = max(k, 2); r < 2 * n; r++) {
    t = 'z';
    for (int i = 1; i <= n; i++) {
      int j = r - i;
      if (j > 0 && j <= n && flag[i][j]) {
        smin(t, s[i][j + 1]);
        smin(t, s[i + 1][j]);
      }
    }
    putchar(t);
    for (int i = 1; i <= n; i++) {
      int j = r + 1 - i;
      if (j > 0 && j <= n && s[i][j] == t) {
        if (i > 1 && flag[i - 1][j]) smax(flag[i][j], 1);
        if (j > 1 && flag[i][j - 1]) smax(flag[i][j], 1);
      }
    }
  }
  return 0;
}
