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#include <stdio.h>
#include <stdlib.h>

#include <iostream>
#include <set>

#define MAX_LEN 200000

using namespace std;

int main(void)
{
  long result = -1;

  static char data [MAX_LEN + 1];

  set<int> pos [2];

  char * beg = data, * end = beg;

  int offs = 0;

  if (scanf("%s", data)) /* antiwarning */
  {
  }

  /* fill pos */

  while (*end)
  {
    pos[*end++ == 'b'].insert(offs++);
  }

  #ifdef DBG
  {set<int>::iterator i; for(i = pos[0].begin(); i != pos[0].end(); ++i) { cerr << *i << " "; } cerr << "\n";}
  {set<int>::iterator i; for(i = pos[1].begin(); i != pos[1].end(); ++i) { cerr << *i << " "; } cerr << "\n";}
  dbgs(data);
  #endif

  if (!((pos[0].size() & 1) && (pos[1].size() & 1)))
  {
    result = 0;

    while (beg < --end)
    {
      set<int> & b = pos[*beg == 'b'];
      set<int> & e = pos[*end == 'b'];

      #ifdef DBG
      dbg(DBG_EOL);
      {set<int>::iterator i; for(i = pos[0].begin(); i != pos[0].end(); ++i) { cerr << *i << " "; } cerr << "\n";}
      {set<int>::iterator i; for(i = pos[1].begin(); i != pos[1].end(); ++i) { cerr << *i << " "; } cerr << "\n";}
      dbgv(beg - data);
      dbgs(data);
      #endif

      if (&b == &e)
      {
        b.erase(b.begin());
        b.erase(--b.end());

        ++beg;
      }
      else
      {
        int beg_len = abs(*b. begin() - *e. begin());
        int end_len = abs(*b.rbegin() - *e.rbegin());

        set<int>::iterator bi;
        set<int>::iterator ei;

        if (beg_len <= end_len)
        {
          bi = b.begin();
          ei = e.begin();

          result += beg_len;
        }
        else
        {
          bi = --b.end();
          ei = --e.end();

          result += end_len;
        }

        swap(data[*bi], data[*ei]);

        b.insert(*ei);
        e.insert(*bi);

        b.erase(bi);
        e.erase(ei);

        ++end;
      }
    }

  }

  printf("%ld\n", result);

  return EXIT_SUCCESS;
}