#pragma region Macros

#pragma GCC target("avx,avx2,fma")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")

#include <bits/extc++.h>
using namespace std;
using namespace __gnu_pbds;
// using namespace __gnu_cxx;

// #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 int INF = 1 << 30;
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);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    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);

		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)];
	}

    int get_sum(int x) {
        return sum[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;
    vector<int> sum;
};

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; }

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

const vector<int> dx = {1, 0};
const vector<int> dy = {0, 1};

int H, W;
bool isvalid(int x, int y) {
    if (x >= 0 && x < H && y >= 0 && y < W) return true;
    else return false;
}

signed main() {
    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<pair<int,int>> A;
    A.pb(make_pair(0, 0));
    cout << S[0][0];

    for (int i = 0; i < H + W - 2; i++) {

        char c = 'z';
        for (auto &p : A) {
            for (int k = 0; k < 2; k++) {
                int ni = p.first + dx[k];
                int nj = p.second + dy[k];

                if (!isvalid(ni, nj)) continue;

                c = min(c, S[ni][nj]);
            }
        }
        cout << c;

        vector<pair<int,int>> B;
        for (auto &p : A) {
            for (int k = 0; k < 2; k++) {
                int ni = p.first + dx[k];
                int nj = p.second + dy[k];

                if (!isvalid(ni, nj)) continue;

                if (S[ni][nj] == c) B.pb(make_pair(ni, nj));
            }
        }
        swap(A, B);
    }
    cout << endl;
}