#pragma region Macros

// #pragma GCC target("avx,avx2,fma")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC target("avx,avx2,fma,sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,tune=native")

#include <bits/extc++.h>
// #include <bits/stdc++.h>
using namespace std;
using namespace __gnu_pbds;
// using namespace __gnu_cxx;
// #include <atcoder/fenwicktree>
// #include <atcoder/segtree>
// #include <atcoder/maxflow>
// #include <atcoder/all>
// using namespace atcoder;

// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision;
// using Bint = mp::cpp_int;

#define TO_STRING(var) # var
#define pb emplace_back
#define int ll
#define endl '\n'

using ll = long long;
using ld = long double;
const ld PI = acos(-1);
const ld EPS = 1e-10;
const ll INFL = 1LL << 61;
const int MOD = 998244353;
// const int MOD = 1000000007;

__attribute__((constructor))
void constructor() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout << fixed << setprecision(15);
}

class UnionFind {
public:

	UnionFind() = default;
    UnionFind(int n) : par(n), 
	    sz(n, 1) { iota(par.begin(), par.end(), 0); }

	int root(int x) {
		if (par[x] == x) return x;
		return (par[x] = root(par[x]));
	}

	bool unite(int x, int y) {
		int rx = root(x);
		int ry = root(y);

        if (rx == ry) return false;
		if (sz[rx] < sz[ry]) swap(rx, ry); // union by size 

		sz[rx] += sz[ry];
		par[ry] = rx;

        return true;
	}

	bool issame(int x, int y) {
		return (root(x) == root(y));
	}

	int size(int x) {
		return sz[root(x)];
	}

    vector<vector<int>> groups(int n) {
        vector<vector<int>> G(n);
        for (int x = 0; x < n; x++) {
            G[root(x)].push_back(x);
        }
		G.erase(
            remove_if(G.begin(), G.end(),
                [&](const vector<int>& v) { return v.empty(); }),
                    G.end());
        return G;
    }

private:
	vector<int> par;
	vector<int> sz;
};

template<int mod> class modint{
public:
    int val = 0;
    modint(int x = 0) { while (x < 0) x += mod; val = x % mod; }
    modint(const modint &r) { val = r.val; } // コピーコンストラクタ

    modint operator -(){ return modint(-val); } // 単項
    modint operator +(const modint &r) { return modint(*this) += r; }
    modint operator -(const modint &r) { return modint(*this) -= r; }
    modint operator *(const modint &r) { return modint(*this) *= r; }
    modint operator /(const modint &r) { return modint(*this) /= r; }

    modint &operator +=(const modint &r) {
        val += r.val;
        if (val >= mod) val -= mod;
        return *this;
    }
    modint &operator -=(const modint &r) {
        if (val < r.val) val += mod;
        val -= r.val;
        return *this;
    }
    modint &operator *=(const modint &r) {
        val = val * r.val % mod;
        return *this;
    }
    modint &operator /=(const modint &r) {
        int a = r.val, b = mod, u = 1, v = 0;
        while (b) {
            int t = a / b;
            a -= t * b; swap(a, b);
            u -= t * v; swap(u, v);
        }
        val = val * u % mod;
        if (val < 0) val += mod;
        return *this;
    }

    bool operator ==(const modint& r) { return this -> val == r.val; }
    bool operator <(const modint& r) { return this -> val < r.val; }
    bool operator !=(const modint& r) { return this -> val != r.val; }
};

using mint = modint<MOD>;

istream &operator >>(istream &is, mint& x) {
    int t; is >> t;
    x = t;
    return (is);
}
ostream &operator <<(ostream &os, const mint& x) {
    return os << x.val;
}

mint modpow(const mint &a, int n) {
    if (n == 0) return 1;
    mint t = modpow(a, n / 2);
    t = t * t;
    if (n & 1) t = t * a;
    return t;
}

int modpow(int x, int N, int mod) {
    int ret = 1;
    while (N > 0) {
        if (N % 2 == 1) ret = ret * x % mod;
        x = x * x % mod;
        N /= 2;
    }
    return ret;
}

int ceil(int x, int y) { return (x > 0 ? (x + y - 1) / y : x / y); }

#pragma endregion

signed main() {
    int H, W;
    cin >> H >> W;
    vector<vector<char>> S(H, vector<char>(W));
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {
            cin >> S[i][j];
        }
    }

    vector<char> ans(H + W - 1, 123);
    vector<vector<vector<int>>> flag(H, vector<vector<int>>(W, vector<int>(2)));

    ans[0] = S[0][0];

    queue<tuple<int,int,int>> qu;
    tuple<int,int,int> t(0, 0, 0);
    qu.push(t);

    while (!qu.empty()) {
        auto T = qu.front();
        qu.pop();
        int i = get<0>(T);
        int j = get<1>(T);
        int k = get<2>(T);

        if (i && k == 0 && S[i - 1][j] != ans[i + j - 1]) continue;
        if (j && k == 1 && S[i][j - 1] != ans[i + j - 1]) continue;

        char c = 'z';
        if (S[i][j] < ans[i + j]) {
            ans[i + j] = S[i][j];
            if (i + 1 < H && !flag[i + 1][j][0]) {
                flag[i + 1][j][0] = true;
                qu.emplace(make_tuple(i + 1, j, 0));
                c = S[i + 1][j];
            }
            if (j + 1 < W && !flag[i][j + 1][1]) {
                flag[i][j + 1][1] = true;
                qu.emplace(make_tuple(i, j + 1, 1));
                c = S[i][j + 1];
            }
        }
        if (S[i][j] == ans[i + j]) {
            if (i + 1 < H && !flag[i + 1][j][0] && S[i + 1][j] <= c) {
                flag[i + 1][j][0] = true;
                qu.emplace(make_tuple(i + 1, j, 0));
                c = min(c, S[i + 1][j]);
            }
            if (j + 1 < W && !flag[i][j + 1][1] && S[i][j + 1] <= c) {
                flag[i][j + 1][1] = true;
                qu.emplace(make_tuple(i, j + 1, 1));
                c = min(c, S[i][j + 1]);
            }
        }
    }
    for (auto &x : ans) {
        cout << x;
    }
    cout << endl;
}