#include <iostream>
#include <cstring>
using namespace std;

char s[260],cuv[260][12];
int n;

int pal(char s[])
{
    int i,j;
    i=0;
    j=strlen(s)-1;
    while(i<j)
    {
        if(s[i]==s[j])
        {
            i++;
            j--;
        }
        else
            return 0;
    }
    return 1;
}

int main()
{
    char*p,sep[]=" ,.", sol[12];
    int i;
    strcpy(sol, "zzzzzzzzzz");
    cin.getline(s,256);
    p=strtok(s,sep);
    while(p!=NULL)
    {
        n++;
        strcpy(cuv[n],p);
        p=strtok(NULL,sep);
    }
    for(i=1;i<=n;i++)
    {
        if(pal(cuv[i]))
        {
            if (strcmp(cuv[i],sol)<0)
                strcpy(sol,cuv[i]);
        }
    }
    if(strcmp(sol,"zzzzzzzzzz")==0)
        cout<<"IMPOSIBIL";
        else
            cout<<sol;
    return 0;
}