#include <iostream>
#include <numeric>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <string>
#include <set>
#include <map>
using namespace std;
#define sp ' '
#define st first
#define nd second
#define pb push_back
#define mp make_pair
#define forn(YY, yy) for(long long int yy = 0; yy < YY; ++yy)
#define prn(XX) cout << XX << endl
#define prs(XX) cout << XX << " "
typedef long long int ll;
typedef unsigned long long int ull;
typedef vector<ll> vll;
typedef vector<vector<ll>> vvll;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
const int MOD = 1e9 + 7;
const int INF = 2e9 + 13;
const int mINF = -2e9 - 13;
const double PI = 3.14159265358979;
const double EPS = 1e-9;
int N, M;
vector<vector<char>> grid;
map<pair<int, int>, bool> visited;
int main(int argc, char **argv){
ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
cin >> N >> M;
grid.resize(N, vector<char>(M));
for(int i = 0; i < N; ++i) for(int j = 0; j < M; ++j) cin >> grid[i][j];
queue<pair<string, pair<int, int>>> q; string s; s += grid[0][0];
q.push(mp(s, mp(0, 0)));
set<string> Set;
while(!q.empty()){
pair<string, pair<int, int>> t = q.front(); q.pop();
int x = t.nd.st, y = t.nd.nd;
if(x == N-1 and y == M-1){ Set.insert(t.st); continue; }
if(x < N-1 and y < M-1){
if(grid[x][y+1] < grid[x+1][y]){
if(!visited[mp(x, y+1)]){
visited[mp(x, y+1)] = true;
q.push(mp(t.st + grid[x][y+1], mp(x, y+1)));
}
}
else if(grid[x][y+1] > grid[x+1][y]){
if(!visited[mp(x+1, y)]){
visited[mp(x+1, y)] = true;
q.push(mp(t.st + grid[x+1][y], mp(x+1, y)));
}
}
else{
if(!visited[mp(x+1, y)]){
visited[mp(x+1, y)] = true;
q.push(mp(t.st + grid[x+1][y], mp(x+1, y)));
}
if(!visited[mp(x, y+1)]){
visited[mp(x, y+1)] = true;
q.push(mp(t.st + grid[x][y+1], mp(x, y+1)));
}
}
}
else if(x >= N-1 and y < M-1){
if(!visited[mp(x, y+1)]){
visited[mp(x, y+1)] = true;
q.push(mp(t.st + grid[x][y+1], mp(x, y+1)));
}
}
else if(x < N-1 and y >= M-1){
if(!visited[mp(x+1, y)]){
visited[mp(x+1, y)] = true;
q.push(mp(t.st + grid[x+1][y], mp(x+1, y)));
}
}
}
cout << *(Set.begin()) << endl;
return 0;
}
//cikisir
