import std;

struct Grid {
    int x, y;

    int opCmp(Grid that) {
        if (this.x != that.x) {
            if (this.x > this.x) return 1;
            else return -1;
        }
        if (this.y != that.y) {
            if (this.y > this.y) return 1;
            else return -1;
        }
        return 0;
    }
}

void main() {
    int H, W;
    readf("%d %d\n", H, W);

    auto S = new string[](H);
    foreach (i; 0 .. H) readf("%s\n", S[i]);

    auto mn = new string[](H+W);
    mn[1] = S[0][0].to!string;
    foreach (i; 2 .. H+W) mn[i] = "{";

    Grid[] list;
    list ~= Grid(0, 0);
    foreach (i; 2 .. H+W) {
        Grid[] nxt;
        foreach (l; list) {
            if (l.x + 1 < H) {
                string T = mn[i-1] ~ S[l.x+1][l.y];
                if (T < mn[i]) {
                    nxt = [Grid(l.x+1, l.y)];
                    mn[i] = T;
                }
                else if (T == mn[i]) {
                    nxt ~= Grid(l.x+1, l.y);
                }
            }

            if (l.y + 1 < W) {
                string T = mn[i-1] ~ S[l.x][l.y+1];
                if (T < mn[i]) {
                    mn[i] = T;
                    nxt = [Grid(l.x, l.y+1)];
                }
                else if (T == mn[i]) {
                    nxt ~= Grid(l.x, l.y+1);
                }
            }
        }

        nxt.sort;
        list = nxt.uniq.array;
    }

    string res = mn.back;
    res.writeln;
}