#include <bits/stdc++.h>
#define inf 2e9
#define all(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair <int, int> pii;
const int N = 2000 + 3;
int n, m, p[N][N], was[N][N], mn[N + N];
char a[N][N];
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
#ifdef LOCAL
freopen("input.txt", "r", stdin);
#endif // LOCAL
cin >> n >> m;
for (int i = 0; i < n; i++)
cin >> a[i];
queue <int> q;
q.push(0);
fill(mn, mn + n + m + 2, inf);
// memset(mn, inf, sizeof mn);
mn[0] = a[0][0] - 'a';
while (!q.empty()){
int x = q.front() / m, y = q.front() % m;
q.pop();
was[x][y] = 1;
if (a[x][y] - 'a' > mn[x + y]) continue;
if (x + 1 < n && !was[x + 1][y]){
p[x + 1][y] = x * m + y;
mn[x + y + 1] = min(mn[x + y + 1], a[x + 1][y] - 'a');
q.push((x + 1) * m + y);
}
if (y + 1 < m && !was[x][y + 1]){
p[x][y + 1] = x * m + y;
mn[x + y + 1] = min(mn[x + y + 1], a[x][y + 1] - 'a');
q.push(x * m + y + 1);
}
}
vector <char> way;
way.push_back(a[n - 1][m - 1]);
int x = (n - 1) * m + m - 1;
do{
x = p[x / m][x % m];
// cout << a[x / m][x % m] << "!\n";
way.push_back(a[x / m][x % m]);
}while (x);
reverse(all(way));
for (auto u: way) cout << u;
}
