#include <cstdio>#include <cstring>#include <cmath>#include <string>#include <iostream>#include <algorithm>#include <queue>#include <set>#include <map>using namespace std;typedef long long ll;#define pii pair<int, int>#define mkpii make_pair<int, int>#define pdi pair<double, int>#define mkpdi make_pair<double, int>#define pli pair<ll, int>#define mkpli make_pair<ll, int>#define rep(i, n) for(int i=0; i<(n); ++i)#define for1(i,a,n) for(int i=(a);i<=(n);++i)#define for2(i,a,n) for(int i=(a);i<(n);++i)#define for3(i,a,n) for(int i=(a);i>=(n);--i)#define for4(i,a,n) for(int i=(a);i>(n);--i)#define CC(i,a) memset(i,a,sizeof(i))#define read(a) a=getint()#define print(a) printf("%d", a)#define dbg(x) cout << (#x) << " = " << (x) << endl#define error(x) (!(x)?puts("error"):0)#define printarr2(a, b, c) for1(_, 1, b) { for1(__, 1, c) cout << a[_][__]; cout << endl; }#define printarr1(a, b) for1(_, 1, b) cout << a[_] << ‘\t‘; cout << endlinline const int getint() { int r=0, k=1; char c=getchar(); for(; c<‘0‘||c>‘9‘; c=getchar()) if(c==‘-‘) k=-1; for(; c>=‘0‘&&c<=‘9‘; c=getchar()) r=r*10+c-‘0‘; return k*r; }const int N=1005;char s1[N], s2[N];int f[N][N], mn[N][N];string p[N][N];int main() {	while(~scanf("%s", s1+1)) {		CC(mn, 0x3f);		int n=strlen(s1+1);		for1(i, 1, n) s2[n-i+1]=s1[i];		for1(i, 1, n) for1(j, 1, n) {			if(s1[i]==s2[j]) f[i][j]=f[i-1][j-1]+1, p[i][j]=p[i-1][j-1]+s1[i];			else {				f[i][j]=max(f[i-1][j], f[i][j-1]);				if(f[i-1][j]==f[i][j-1]) p[i][j]=min(p[i-1][j], p[i][j-1]);				else if(f[i-1][j]>f[i][j-1]) p[i][j]=p[i-1][j];				else p[i][j]=p[i][j-1];			}		}		int len=(f[n][n]+1)>>1;		string ans1(p[n][n].begin(), p[n][n].begin()+len);		string ans2(ans1.rbegin()+(f[n][n]&1), ans1.rend());		printf("%s\n", (ans1+ans2).c_str());	}	return 0;}