#include<bits/stdc++.h>
#include<atcoder/all>
using namespace atcoder;
//using mint = modint1000000007;
using mint = modint998244353;
//g++ -I/opt/ac-library ./**.cpp
using namespace std;
using ll = long long;
using ull = unsigned long long;
//const long long MOD = 1000000007;
const long long MOD = 998244353;
const long double PI = 3.14159265358979;
const long long INF = 1LL<<60;
template <typename T> bool chmax(T &a, const T& b){if(a < b){a = b;return true;}return false;}
template <typename T> bool chmin(T &a, const T& b){if(a > b){a = b;return true;}return false;}
#define deb(var) do{cout << #var << " : "; view(var);}while(0)
template<typename T> void view(T e){cout << e << endl;}
void view(vector<string>& v){cout << endl;for(auto& s :v){view(s);}cout << endl;}
template<typename T> void view(vector<T>& v){for(auto& e :v){cout << e << " ";}cout << endl;}
template<typename T> void view(vector<vector<T>>& vv){cout << endl;for(auto& v:vv){view(v);}}
ll gcd(ll a, ll b){if (b == 0) return a;else return gcd(b, a % b);}
ll lcm(ll x,ll y){return ll(x/gcd(x,y))*y;}
template<typename T> using min_priority_queue = priority_queue<T, vector<T>, greater<T>>;
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define int long long


int32_t main() {
    int h, w; cin >> h >> w;
    vector<string> s(h);
    for (int i=0; i<h; i++) {
        cin >> s[i];
    }
    vector<vector<bool>> dp(h, vector<bool> (w, false));
    dp[0][0] = true;
    for (int k=0; k<h + w - 3; k++) {
        char mn = 'z';
        for (int i=0; i<=k; i++) {
            int j = k - i;
            if (i >= h || j >= w) continue;
            if (!dp[i][j]) continue;
            if (i < h - 1) chmin(mn, s[i + 1][j]);
            if (j < w - 1) chmin(mn, s[i][j + 1]);
        }
        for (int i=0; i<=k; i++) {
            int j = k - i;
            if (i >= h || j >= w) continue;
            if (!dp[i][j]) continue;
            if (i < h - 1 && s[i + 1][j] == mn) dp[i + 1][j] = true;
            if (j < w - 1 && s[i][j + 1] == mn) dp[i][j + 1] = true;
        }
    }
    
    int i = h - 1, j = w - 1;
    string ans;
    while (!(i == 0 && j == 0)) {
        ans.push_back(s[i][j]);
        if (i > 0 && dp[i - 1][j]) {
            i--;
        }
        else {
            j--;
        }
    }
    ans.push_back(s[0][0]);
    reverse(all(ans));
    cout << ans << "\n";
}