#include <algorithm>
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define ld long double
#define ff first
#define ss second
#define ln "\n"
#define mp make_pair
#define pb push_back
#define INF (ll)2e18
#define MOD (ll)(1e9+7)
struct SegTree{
struct node{
ll mx, mn, upd, tag;
};
vector<node> t;
ll sz, rsz;
SegTree(ll N){
sz=N*4; rsz=N; t.resize(sz);
}
void preprocess(ll tl, ll tr, ll v){
if (t[v].tag){
t[v].tag=0; t[v].mx=t[v].mn=t[v].upd;
if (tl!=tr){
t[v*2].upd=t[v*2+1].upd=t[v].upd;
t[v*2].tag=t[v*2+1].tag=1;
}
}
}
node merge(node l, node r){
return {max(l.mx, r.mx), min(l.mn, r.mn), 0, 0};
}
void update(ll tl, ll tr, ll v, ll l, ll r, ll x){
preprocess(tl, tr, v);
if (tl==l and tr==r){
t[v].upd=x; t[v].tag=1;
preprocess(tl, tr, v);
}else if (l>r) return;
else{
ll tm = (tl+tr)/2;
update(tl, tm, v*(2), l, min(r, tm), x);
update(tm+1, tr, v*2+1, max(l, tm+1), r, x);
t[v] = merge(t[v*2], t[v*2+1]);
}
}
void update(ll l, ll r, ll x){
// cout << l << " - " << r << " <- " << x << ln;
update(0, rsz-1, 1, l, r, x);
}
ll query(ll tl, ll tr, ll v, ll l, ll r, ll x){
preprocess(tl, tr, v);
if (l<=tl and tr<=r and t[v].mx<x){
return tr-tl+1;
}else if (l>tr or r<tl or (t[v].mn>=x)){
return 0;
}else{
ll tm = (tl+tr)/2;
return query(tl, tm, v*2, l, r, x)+query(tm+1, tr, v*2+1, l, r, x);
}
}
ll query(ll l, ll r, ll x){
// cout << l << " - " << r << " -> " << x << " = ";
// cout << query(0, rsz-1, 1, l, r, x) << ln;
return query(0, rsz-1, 1, l, r, x);
}
};
array<ll, 4> eval(string &s, ll l){
if (s[l]=='?'){
return {0, 0, l+1, 1};
}else{
ll il=l; l+=4;
array<ll, 4> ret1 = eval(s, l); l=ret1[2]+1;
array<ll, 4> ret2 = eval(s, l); l=ret2[2]+1;
if (s[il+1]=='a'){
return {ret1[0]+ret2[0]+1, min(ret1[1], ret2[1]), l, ret1[3]+ret2[3]};
}else{
return {min(ret1[0], ret2[0]), ret1[1]+ret2[1]+1, l, ret1[3]+ret2[3]};
}
}
}
void solve(){
string s; cin >> s;
array<ll, 4> res=eval(s, 0);
cout << res[3]-(res[1]+res[0]) << ln;
}
/*
*/
int main(){
ios_base::sync_with_stdio(false); cin.tie(nullptr);
#ifdef LOCAL
auto start = chrono::high_resolution_clock::now();
#endif
ll t=1;
// cin >> t;
for (ll __c=1; __c<=t; __c++) solve();
#ifdef LOCAL
auto duration = chrono::duration_cast<chrono::microseconds>(chrono::high_resolution_clock::now() - start);
cout << setprecision(0) << fixed << "time: " << (double)duration.count()/1000.0 << " milliseconds" << endl;
#endif
}
