#include<bits/stdc++.h>
using namespace std;
using ll = long long;

int main(){
    int h, w;
    cin >> h >> w;
    vector<string> A(h);
    for(auto &&s:A)cin >> s;
    vector<vector<bool>> dp(h, vector<bool>(w));
    dp[0][0] = true;
    string ans;
    ans += A[0][0];
    for(int len = 0; len < h + w - 2; len++){
        vector<tuple<char,int,int>> q;
        for(int y = 0; y <= len; y++){
            int x = len - y;
            if(x < 0 || y >= h || y < 0 || x >= w)continue;
            if(!dp[y][x])continue;
            if(y + 1 < h)q.emplace_back(A[y + 1][x], y + 1, x);
            if(x + 1 < w)q.emplace_back(A[y][x + 1], y, x + 1);
        }
        sort(q.begin(), q.end());
        char c, o = get<0>(q[0]);
        int y, x;
        ans += o;
        for(int i = 0; i < q.size() && get<0>(q[i]) == o; i++){
            tie(c, y, x) = q[i];
            dp[y][x] = true;
        }
    }
    cout << ans << '\n';
}