#include <bits/stdc++.h>
using namespace std;
const int MOD = 1e9 + 7;
const int N = 1e6 + 5;
const int K = (1 << 17) + 10;
const long long int INF = 2e18 + 500;
const int inf = 0x7fffffff;
const int SQRT = 500;
const int lgN = 20;
int main(int argc, char const *argv[]) {
  cin.sync_with_stdio(0);
  cin.tie(0);
  int n;
  cin >> n;
  stack<int> S;
  int ans = 0;
  int req = 0;
  for (int i = 0; i < 2 * n; i++) {
    string s;
    cin >> s;
    if (s[0] == 'a') {
      int x;
      cin >> x;
      S.push(x);
    } else {
      req += 1;
      if (S.size() == 0) {
        continue;
      } else {
        int x = S.top();
        if (x == req) {
          S.pop();
        } else {
          ans += 1;
          S = stack<int>();
        }
      }
    }
  }
  cout << ans << endl;
  return 0;
}
