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w = list(input())
na = w.count('a');
nb = len(w) - na
if len(w) % 2 == 0 and na % 2 != 0:
    print(-1);
    exit(0)
rc = lc = 0
moves = 0    
for i in range(len(w)//2):
    if rc == 0:
        while w[-i-rc-1] == w[-i-1]:
            rc+=1
    if lc == 0:        
        while w[lc] == w[0]:
            lc+=1
    #print(*w,sep='',end='')
    #print(" rc={} lc={}".format(rc,lc))
    if w[i] != w[-i-1]:
        if rc < lc:
            w[-i-rc-1]=w[-i-1]
            w[-i-1]=w[i]
            moves += rc
        else:
            w[i+lc]=w[i]
            w[i]=w[-i-1]
            moves += lc
    rc-=1
    lc-=1
    #print(*w,sep='',end='')
    #print(" moves={}".format(moves))
print(moves)