a=list(raw_input())
alp={"a":0,"b":0,"c":0,"d":0,"e":0,"f":0,"g":0,"h":0,"i":0,"j":0,"k":0,"l":0,"m":0,"n":0,"o":0,"p":0,"q":0,"r":0,"s":0,"t":0,"u":0,"v":0,"w":0,"x":0,"y":0,"z":0}

for i in a:
    alp[str(i)]+=1

ans=1
for i in alp:
    if alp[i]%2!=0:
        ans+=1
print ans