#include <iostream>
#include <cstring>
using namespace std;
bool palindrom(char *p)
{
for(int i = 0 , j = strlen(p) - 1 ; i < j ; i ++, j --)
if(p[i] != p[j])
return false;
return true;
}
int main()
{
char s[256], cmin[11];
cin.getline(s, 256);
char * p = strtok(s , " .,");
int cnt = 0;
while(p)
{
if(palindrom(p))
{
cnt ++;
if(cnt == 1)
strcpy(cmin , p);
else
if(strcmp(p , cmin) < 0)
strcpy(cmin , p);
}
p = strtok(NULL , " .,");
}
if(cnt > 0)
cout << cmin;
else
cout << "IMPOSIBIL";
return 0;
}
