#pragma GCC optimize("O3,inline")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
using namespace std;
using ll = long long;
#define int ll
using ii = pair<int, int>;
using iii = pair<ii, int>;
constexpr int INF = 1e18 + 5;
constexpr int MAXN = 1000000 + 5;
constexpr int MOD = 1e9 + 7;
string s;
int ctr = 0;
ii dfs() {
if (s[ctr] == '?') return ii{0, 0};
bool ismax = s[ctr] == 'a';
ctr++;
ii ch1 = dfs();
ctr++;
ii ch2 = dfs();
ii ans;
if (ismax) {
ans.first = ch1.first + ch2.first + 1;
ans.second = min(ch1.second, ch2.second);
}
else {
ans.first = min(ch1.first, ch2.first);
ans.second = ch1.second + ch2.second + 1;
}
return ans;
}
signed main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
string t; cin >> t;
int N = 0;
for (char c : t) {
if (c != 'a' && c != 'i' && c != '?') continue;
if (c == '?') N++;
s += c;
}
ii res = dfs();
cout << (N - res.first - res.second);
}
