#include <fstream>
#include<cstring>

using namespace std;

ifstream in("palm.in");
ofstream out("palm.out");

char q[501];
int v[10];

int main()
{
    int n,i,max=0,d=0;
    in.getline(q,501);
    n=strlen(q);
    for(i=1;i<=n;i++)
        v[q[i-1]-'a'+1]++;
	for(i=1;i<=26;i++)
		if(v[i]%2==0)
			max+=v[i];
		else
		{
			max+=v[i]-1;
			if(d==0)
			{
				d=1;
				max++;
			}
		}
    out<<max<<'\n';
    return 0;
}