//```// YF YUSUF
#include <bits/stdc++.h>
using namespace std;
#ifdef YF_CHECK
bool LOCAL = 1;
#else
#pragma GCC target("avx2")
#pragma GCC optimize ("unroll-loops")
#pragma GCC optimize ("inline")
#pragma GCC optimize ("Ofast")
#pragma GCC optimize ("O3")
bool LOCAL = 0;
#endif
using i128 = __int128;
using ll = long long;
using ld = long double;
using vll = vector <ll>;
using mll = map <ll,ll>;
using pll = pair <ll,ll>;
using vvl = vector <vll>;
using vpll = vector <pll>;
template<class T>T MIN(T&a,T b){a=min(a,b);return a;}
template<class T>T MAX(T&a,T b){a=max(a,b);return a;}
#define rall(a) a.rbegin(),a.rend()
#define all(a) a.begin(),a.end()
#define sgr v+v+1,(tl+tr)/2+1,tr
#define sgl v+v,tl,(tl+tr)/2
#define pb push_back
#define S second
#define F first
mt19937_64 MT(time(0));
ll BP(ll a,ll b,ll mod=1e9+7){
if(b==0)return 1;
ll q=BP(a,b/2,mod);
return ((q*q)%mod*(b%2?a:1ll))%mod;
}
ll f(ll x){return x*(x+1)/2;}
ll dup(ll a,ll b){return (a+b-1)/b;}
ll lcm(ll a,ll b){return a/__gcd(a,b)*b;}
ll invf(ll x){return (-1+sqrt(1+8*x))/2;}
ll lg(ll x){return (x ? 63 - __builtin_clzll(x) : -1);}
const ll mod=998244353;
const ll INF=1e18;
const ll inf=1e9+7;
const ll N =1e6+7;
char a[3333][3333];
ll slow_cnk(ll n,ll k){
MIN(k, n-k);
ll ans = 1;
for (int i = 1; i <= k; i++)
ans = (ans * (n-k+i) / i);
return ans;
}
ll get(ll n,ll m){
return slow_cnk(n+m-2, min(n-1, m-1));
}
ll n, m, k;
ll GET(ll i,ll j){
return get(n-i+1, m-j+1);
}
void YF_MAIN(ll TEST){
cin>>n>>m;
for(int i=1;i<=n;i++){
for(int j=1;j<=k;j++){
a[i][j] = 'A';
cin>>a[i][j];
}
}
cin>>k;
ll i = 1, j = 1;
vpll path = {{i, j}};
while(i<=n && j<=k){
if(i == n && j == k)break;
bool was = 0;
vector<ll>js = {j, j+1};
for(int k=i+j+1;k<=n+m;k++){
vector<array<ll, 4>>v;
for(auto J : js){
int I = k - J;
if(!(1<=I && I<=n) || !(1<=J&&J<=m))continue;
v.pb({a[I][J], GET(I, J), J, I});
}
if(v.empty()){
cout<<"WHAT";
break;
}
sort(all(v));
ll now = 0;
for(auto [c, x, I, J] : v){
if(now + x >= k){
} else now += x;
}
}
if(!was)break;
}
if(path.size() != n + n - 1)cout<<"SIZE SMALLER\n";
pll last = {1, 0};
int cnt = 0;
for(auto [i, j] : path){
cout<<a[i][j];
}
}
const bool TECT=0;
const bool FLSH=1;
const ll SN=1e0 + 7; ll SM[SN];
const ll FN=1e0 + 7; ll FACT[FN], inv[FN], FMOD=inf;
ll PNK(ll n,ll k){return FACT[n] *inv[n-k]%FMOD;}
ll CNK(ll n,ll k){return PNK(n,k)*inv[k ]%FMOD;}
void BEFORE(){
for(ll i=2;i<SN;i++){
if(SM[i])continue;
for(ll j=i;j<SN;j+=i)
MAX(SM[j],i);
}
FACT[0]=inv[0]=1;
for(int i=1;i<FN;i++){
FACT[i]=FACT[i-1]*i%FMOD;
inv[i]=BP(FACT[i],FMOD-2,FMOD);
}
}
signed main(){
// freopen(("input.txt"),"r",stdin);freopen(("output.txt"),"w",stdout);
if(FLSH){
ios_base::sync_with_stdio(0);
cout.setf(ios::fixed);
cout.precision(0);
cout.tie(0);
cin.tie(0);
}
int TEST=1;
if(TECT)
cin>>TEST;
BEFORE();
for(int T=1;T<=TEST;T++){
// cout<<"Case "<<T<<": ";
YF_MAIN(T);
cout<<(T==TEST ? "" : "\n");
}
return 0;
}
// YF YUSUF ```
