#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
 typedef unsigned long long ull;
 typedef pair < int, int > pii;
 const int inf = 0x3f3f3f3f;
 const double eps = 1e-8;
 const int mod = 1000000007;
 const double pi = acos(-1.0);
 inline void gn(long long & x) {
     int sg = 1;
     char c;
     while (((c = getchar()) < '0' || c > '9') && c != '-');
     c == '-' ? (sg = -1, x = 0) : (x = c - '0');
     while ((c = getchar()) >= '0' && c <= '9') x = x * 10 + c - '0';
     x *= sg;
 }
 inline void gn(int & x) {
     long long t;
     gn(t);
     x = t;
 }
 inline void gn(unsigned long long & x) {
     long long t;
     gn(t);
     x = t;
 }
 ll gcd(ll a, ll b) {
     return a ? gcd(b % a, a) : b;
 }
 ll powmod(ll a, ll x, ll mod) {
     ll t = 1 ll;
     while (x) {
         if (x & 1) t = t * a % mod;
         a = a * a % mod;
         x >>= 1;
     }
     return t;
 }
 const int maxn = 6e5 + 5;
 int a[maxn];
 stack < int > st;
 char op[40];
 int n;
 int main() {
     gn(n);
     int nowdo = 0;
     int ans = 0;
     for (int i = 1; i <= 2 * n; ++i) {
         scanf("%s", op);
         if (op[0] == 'a') {
             int x;
             gn(x);
             st.push(x);
             a[x] = 1;
         } else {
             nowdo++;
             if (a[nowdo] == 2 && st.empty()) continue;
             else if (nowdo == st.top()) st.pop();
             else {
                 ++ans;
                 while (!st.empty()) {
                     int x = st.top();
                     a[x] = 2;
                     st.pop();
                 }
             }
         }
     }
     cout << ans << endl;
     return 0;
 }