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124 | #include <bits/stdc++.h>
#define INF 2147483647
#define LINF 9223372036854775807
#define NINF -2147483648
#define NLINF -9223372036854775808
#define M 1000000007
#define M1 998244353
#define A 26
#define K 31
#define P 2137
using namespace std;
using db=double;
using dbl=long double;
using ll=long long;
using pi=pair<int,int>;
using pl=pair<ll,ll>;
using vi=vector<int>;
using vl=vector<ll>;
using gr=vector<vector<int> >;
using grl=vector<vector<ll> >;
#define fp(x, a, b) for (int (x) = (a); (x) < (b); (x)++)
#define f(x, n) for (int (x) = 0; (x) < (n); (x)++)
#define fnp(x, a, b) for (int (x) = (b) - 1; (x) >= (a); (x)--)
#define fn(x, n) for (int (x) = (n - 1); (x) >= 0; (x)--)
#define sgn(x) (x) > 0 ? 1 : (x) == 0 ? 0 : -1
#define gcd(a, b) __gcd((a), (b))
#define lcm(a, b) (a) * (b) / gcd((a), (b))
#define x first
#define y second
#define mp make_pair
#define pb push_back
#define s(x) x.size()
#define all(x) x.begin(), x.end()
#define ans(x) cout<<(x)<<"\n"
#define yes cout<<"YES\n";
#define no cout<<"NO\n";
#define fl cout.flush()
#define debarr(x, n) f(i, (n)){cout<<(x)[i]<<" ";}cout<<"\n"
#define debgr(x, n) f(i, (n)){f(j, s((x)[i])){cout<<(x)[i][j]<<" ";}cout<<"\n";}
mt19937 rnd(chrono::high_resolution_clock::now().time_since_epoch().count());
void input();
void compute();
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int T = 1;
//cin >> T;
while(T--)
{
input();
compute();
}
return 0;
}
#define N 5001
string s;
int n;
ll w;
int cntA;
int cntB;
int d;
vi aPos;
void input()
{
cin >> s;
n = s(s);
w = 0;
cntA = 0;
cntB = 0;
d = 0;
}
void compute()
{
f(i, n) {
if(s[i] == 'a') {
cntA++;
if(i <= n / 2) {
d++;
}
} else {
cntB++;
}
}
if((n % 2 == 0) && (cntA % 2 == 1 || cntB % 2 == 1)) {
ans(-1);
return;
}
if(n % 2 == 1 && (cntA % 2 == 1 && cntB % 2 == 1)) {
ans(-1);
return;
}
if(d > s(aPos) / 2) {
reverse(all(s));
}
f(i, s(s)) {
if(s[i] == 'a') {
aPos.pb(i);
}
}
f(i, s(aPos)) {
if(i >= s(aPos) / 2) {
break;
}
int k = aPos[i] - (n - aPos[s(aPos) - i - 1] - 1);
if(k < 0) {
k *= -1;
}
w += k;
}
ans(w);
}
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