#include <bits/stdc++.h>
#define ff first
#define ss second
#define endl '\n'
using namespace std;
const long long INF = (long long) 1e18;
const int mod = (int) 1e9+7;
const int MAXN = (int) 2e3+5;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
ll n, m;
char grid[MAXN][MAXN];
queue<pii> q;
vector<pii> yer[26];
bool marked[MAXN][MAXN];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);cout.tie(nullptr);
#ifdef Local
freopen("../IO/int.txt","r",stdin);
freopen("../IO/out.txt","w",stdout);
#endif
cin>>n>>m;
for(int i = 0; i < n; i++){
for(int j = 0; j < m; j++){
cin>>grid[i][j];
}
}
string cur;
q.push({0, 0});
cur += grid[0][0];
//cout<<cur<<endl;
while(!q.empty()){
int curR = q.front().ff, curC = q.front().ss;
//cout<<curR<<" "<<curC<<endl;
if(curR == n - 1 && curC == m - 1){
break;
}
q.pop();
if(curR + 1 < n && !marked[curR + 1][curC]){
yer[(int)(grid[curR + 1][curC] - 'a')].push_back({curR + 1, curC});
marked[curR + 1][curC] = 1;
}
if(curC + 1 < m && !marked[curR][curC + 1]){
yer[(int)(grid[curR][curC + 1] - 'a')].push_back({curR, curC + 1});
marked[curR][curC + 1] = 1;
}
if(q.empty()){
int i = 0;
for(; i < 26; i++){
if(yer[i].empty()) continue;
cur = cur + (char)('a' + i);
for(pii j: yer[i]){
q.push(j);
}
yer[i].clear();
break;
}
for(;i < 26;i++){
yer[i].clear();
}
}
}
cout<<cur<<endl;
#ifdef Local
cout<<endl<<fixed<<setprecision(2)<<1000.0 * clock() / CLOCKS_PER_SEC<< " milliseconds ";
#endif
}
