#include <bits/stdc++.h>
#define int long long
#define vec vector
#pragma GCC optimize("O2")
#define endl " \n"
#define all(x) x.begin(),x.end()
using namespace std;
int32_t main()
{
ios::sync_with_stdio(false);
cin.tie(nullptr);
cout.precision(0);
cout<<fixed;
int n,m;
cin>>n>>m;
vec<string> x(n);
for (int i = 0; i < n; i++)
{
cin>>x[i];
}
queue<pair<int,int>> q;
vec<vec<int>> p(n,vec<int>(m));
vec<vec<bool>> vis(n,vec<bool>(m,0));
p[0][0]=0;
set<pair<char,pair<int,int>>> qq;
qq.insert({x[0][0],{0,0}});
int r=1;
while(!qq.empty()){
//cout<<"round:"<<r++<<endl[1];
int last=qq.begin()->first;
while((!qq.empty())&&qq.begin()->first==last)
{
q.emplace(qq.begin()->second.first,qq.begin()->second.second);
//cout<<qq.begin()->second.first<<" "<<qq.begin()->second.second<<endl[1];
qq.erase(qq.begin());
}
while((!qq.empty()))
{
qq.erase(qq.begin());
}
while(!q.empty())
{
auto [xx,yy]=q.front();
vis[yy][xx]=1;
if((yy-1>=0&&vis[yy-1][xx])&&(xx-1>=0&&vis[yy][xx-1]))
p[yy][xx]=-1;else
if((yy-1>=0&&vis[yy-1][xx]))
p[yy][xx]=1;else
if((xx-1>=0&&vis[yy][xx-1]))
p[yy][xx]=-1;
q.pop();
if(xx+1<m&&yy+1<n)
{
if(x[yy][xx+1]==x[yy+1][xx])
{
qq.insert({x[yy][xx+1],{xx+1,yy}});
qq.insert({x[yy+1][xx],{xx,yy+1}});
}
if(x[yy][xx+1]>x[yy+1][xx])
{
qq.insert({x[yy+1][xx],{xx,yy+1}});
}
if(x[yy][xx+1]<x[yy+1][xx])
{
qq.insert({x[yy][xx+1],{xx+1,yy}});
}
}else
{
if(xx+1<m)
{
qq.insert({x[yy][xx+1],{xx+1,yy}});
}
if(yy+1<n)
{
qq.insert({x[yy+1][xx],{xx,yy+1}});
}
}
}
}
string res;
int xx=m-1,yy=n-1;
while(p[yy][xx])
{
res.push_back(x[yy][xx]);
if(p[yy][xx]<0)
xx--;
else
yy--;
}
res.push_back(x[yy][xx]);
reverse(all(res));
cout<<res<<endl[1];
return 0;
}
