#include<bits/stdc++.h>
using namespace std;
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int H,W;
    cin>>H>>W;
    vector<string>S(H);
    for(int i=0;i<H;i++)cin>>S[i];
    set<pair<int,int>>cand;
    cand.insert(make_pair(0,0));
    string T(1,S[0][0]);
    for(int t=0;t<H+W-2;t++)
    {
        char min_char='z';
        for(auto[r,c]:cand)
        {
            if(r+1<H)min_char=min(min_char,S[r+1][c]);
            if(c+1<W)min_char=min(min_char,S[r][c+1]);
        }
        T.push_back(min_char);
        set<pair<int,int>>tmp;
        for(auto[r,c]:cand)
        {
            if(r+1<H&&S[r+1][c]==min_char)tmp.insert(make_pair(r+1,c));
            if(c+1<W&&S[r][c+1]==min_char)tmp.insert(make_pair(r,c+1));
        }
        cand=tmp;
    }
    cout<<T<<endl;
}