#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using T = tuple<ll, ll, ll>;
#define int long long
#define Base 41
#define sz(a) (int)a.size()
#define FOR(i, a, b) for ( int i = a ; i <= b ; i++ )
#define FORD(i, a, b) for (int i = b; i >= a; i --)
#define REP(i, n) for (int i = 0; i < n; ++i)
#define REPD(i, n) for (int i = n - 1; i >= 0; --i)
#define all(x) x.begin() , x.end()
#define pii pair<int , int>
#define fi first
#define se second
#define Lg(x) 31 - __builtin_clz(x)
#define MASK(i) (1LL << (i))
#define BIT(x, i) (((x) >> (i)) & 1)
constexpr ll LINF = (1ll << 60);
constexpr int INF = (1ll << 30);
constexpr int MAX = 2e3 + 5;
constexpr int Mod = 1e9 + 7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
void setupIO(){
#define name "Whisper"
//Phu Trong from Nguyen Tat Thanh High School for gifted student
srand(time(NULL));
cin.tie(nullptr)->sync_with_stdio(false); cout.tie(nullptr);
//freopen(name".inp", "r", stdin);
//freopen(name".out", "w", stdout);
cout << fixed << setprecision(10);
}
template <class X, class Y>
bool minimize(X &x, const Y &y){
X eps = 1e-9;
if (x > y + eps) {x = y; return 1;}
return 0;
}
template <class X, class Y>
bool maximize(X &x, const Y &y){
X eps = 1e-9;
if (x + eps < y) {x = y; return 1;}
return 0;
}
int N, M;
char a[MAX][MAX];
int vis[MAX][MAX];
pii ret[MAX][MAX];
const int dx[] = {1, 0};
const int dy[] = {0, 1};
void Whisper(){
cin >> N >> M;
FOR(i, 1, N) FOR(j, 1, M) cin >> a[i][j];
vector<pii> q;
q.emplace_back(1, 1);
vis[1][1] = 1;
while (!q.empty()){
auto[x, y] = q.back();
cout << a[x][y];
char cur = 'z';
vector<pii> nxt;
for (auto&[rx, ry] : q){
REP(dir, 2){
int nx = rx + dx[dir];
int ny = ry + dy[dir];
if (nx > N || ny > M || vis[nx][ny]) continue;
cur = min(cur, a[nx][ny]);
}
}
for (auto&[rx, ry] : q){
REP(dir, 2){
int nx = rx + dx[dir];
int ny = ry + dy[dir];
if (nx > N || ny > M || vis[nx][ny] || a[nx][ny] != cur) continue;
nxt.emplace_back(nx, ny);
vis[nx][ny] = 1;
}
}
swap(q, nxt);
}
}
signed main(){
setupIO();
int Test = 1;
// cin >> Test;
for ( int i = 1 ; i <= Test ; i++ ){
Whisper();
if (i < Test) cout << '\n';
}
}
