#pragma GCC target("avx2")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")


#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
typedef long long ll;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n) - 1; i >= 0; i--)
#define repk(i, k, n) for (int i = k; i < (int)(n); i++)
#define all(v) v.begin(), v.end()
#define mod1 1000000007
#define mod2 998244353
#define vi vector<int>
#define vs vector<string>
#define vc vector<char>
#define vl vector<ll>
#define vvi vector<vector<int>>
#define vvc vector<vector<char>>
#define vvl vector<vector<ll>>
#define vvvi vector<vector<vector<int>>>
#define vvvl vector<vector<vector<ll>>>
#define pii pair<int,int>
#define pil pair<int,ll>
#define pli pair<ll, int>
#define pll pair<ll, ll>
#define vpii vector<pair<int,int>>
#define vpll vector<pair<ll,ll>>
#define vvpii vector<vector<pair<int,int>>>
#define vvpll vector<vector<pair<ll,ll>>>

template<typename T> void debug(T e){
    cerr << e << endl;
}

template<typename T> void debug(vector<T> &v){
    rep(i, v.size()){
        cerr << v[i] << " ";
    }
    cerr << endl;
}

template<typename T> void debug(vector<vector<T>> &v){
    rep(i, v.size()){
        rep(j, v[i].size()){
            cerr << v[i][j] << " ";
        }
        cerr << endl;
    }
}

template<typename T> void debug(vector<pair<T, T>> &v){
    rep(i,v.size()){
        cerr << v[i].first << " " << v[i].second << endl;
    }
}

template<typename T> void debug(set<T> &st){
    for (auto itr = st.begin(); itr != st.end(); itr++){
        cerr << *itr << " ";
    }
    cerr << endl;
}

template<typename T> void debug(multiset<T> &ms){
    for (auto itr = ms.begin(); itr != ms.end(); itr++){
        cerr << *itr << " ";
    }
    cerr << endl;
}

template<typename T> void debug(map<T, T> &mp){
    for (auto itr = mp.begin(); itr != mp.end(); itr++){
        cerr << itr->first << " " << itr->second << endl;
    }
}

void debug_out(){
    cerr << endl;
}

template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T){
    cerr << H << " ";
    debug_out(T...);
}


int main(){
    int H, W;
    cin >> H >> W;
    vector<string> S(H);
    rep(i,H) cin >> S[i];
    vector<vector<int>> vec(H, vector<int>(W));
    rep(i,H){
        rep(j,W){
            vec[i][j] = (int)(S[i][j] - 'a');
        }
    }
    string ans = "";
    vector<vector<bool>> can(H, vector<bool>(W, false));
    can[0][0] = true;
    ans += S[0][0];
    for (int i = 0; i < H + W - 2; i++){
        int min_vec = 27;
        for (int j = 0; j < H; j++){
            if (i - j < 0 || i - j >= W){
                continue;
            }
            if (can[j][i - j]){
                if (j + 1 < H){
                    min_vec = min(min_vec, vec[j + 1][i - j]);
                }
                if (i - j + 1 < W){
                    min_vec = min(min_vec, vec[j][i - j + 1]);
                }
            }
        }
        for (int j = 0; j < H; j++){
            if (i - j < 0 || i - j >= W){
                continue;
            }
            if (can[j][i - j]){
                if (j + 1 < H){
                    if (vec[j + 1][i - j] == min_vec){
                        can[j + 1][i - j] = true;
                    }
                }
                if (i - j + 1 < W){
                    if (vec[j][i - j + 1] == min_vec){
                        can[j][i - j + 1] = true;
                    }
                }
            }
        } 
        //debug(min_vec);
        ans += (char)(min_vec + 'a');
    }
    //debug(can);
    cout << ans << endl;
}