#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int H,W; cin >> H >> W;
    vector<string> S(H);
    rep(i,H) cin >> S[i];

    string T = "";
    T += S[0][0];
    vector<pair<int,int>> can = {{0, 0}};
    while(int(T.size()) < H + W - 1) {
        char mi = 'z';
        vector<pair<int,int>> nt;
        for(auto [x, y] : can) {
            if(x + 1 < H) {
                if(S[x + 1][y] < mi) {
                    mi = S[x + 1][y];
                    nt = {{x + 1, y}};
                } else if(S[x + 1][y] == mi) {
                    nt.push_back({x, y});
                }
            }
            if(y + 1 < W) {
                if(S[x][y + 1] < mi) {
                    mi = S[x][y + 1];
                    nt = {{x, y + 1}};
                } else if(S[x][y + 1] == mi) {
                    nt.push_back({x, y});
                }
            }
        }
        sort(nt.begin(), nt.end());
        nt.erase(unique(nt.begin(), nt.end()), nt.end());
        swap(can, nt);
        T += mi;
    }

    cout << T << endl;
}