#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i < (int)(n+1); i++)
#define repn(i,a,b) for (int i = (int)(a) ; i < (int)(b+1); i++)
#define repv(i, n) for (int i = (int)(n-1); i >= 0; i--)
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define sz(x) ((int)(x).size())
#define cauto const auto&
#define bit(n) (1LL<<(n))
#define uni(v) v.erase( unique(v.begin(), v.end()), v.end() );
using ll = long long;
using P = pair<int,int>;
using PP = pair<int, P>;
using Graph = vector<vector<int>>;
using mint = modint998244353;
#ifdef LOCAL
    #include <debug.hpp>
    #define debug(...) debug::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
    #define debug(...) (static_cast<void>(0))
#endif

int main(){
    int h,w;
    cin >> h >> w;
    vector<string> s(h);
    rep(i,h) cin >> s[i];
    int start_x = 0, start_y = 0;
    string ans = "";
    auto dfs = [&](auto&& dfs, int x, int y) -> void {
        ans += s[y][x];
        if(x == w-1 && y == h-1) return;
        if(x == w-1){
            dfs(dfs, x, y+1);
        } else if(y == h-1){
            dfs(dfs, x+1, y);
        } else {
            if(s[y][x+1] < s[y+1][x]){
                dfs(dfs, x+1, y);
            } else {
                dfs(dfs, x, y+1);
            }
        }
    };
    dfs(dfs, start_x, start_y);
    cout << ans << endl;
    return 0;
}