#include<stdio.h>
#include<string.h>
FILE*f=fopen("palm.in","r");
FILE*g=fopen("palm.out","w");
int v[27],y,e,i,j,t,n,p;
char q[501];
int main (){
	n=26;
	fscanf(f,"%s",q);
	for(i=0;i<strlen(q);i++)
		v[q[i]-96]++;
	y=n;
	p=y;
	while(v[p]%2!=1)
		p--;
	for(i=1;i<=p;i++){
		if(v[i]>1){
			if(v[i]%2==0)
				e+=v[i];
			if((i==p)&&(v[i]%2==1))
				e+=v[p];
			if((i!=p)&&(v[i]%2==1))
				e+=v[i]-v[i]%2;
		}
		if((i==p)&&(v[i]==1))
			e+=v[i];
	}
	fprintf(g,"%d",e);
	return 0;
}