// Homura Akemi a.k.a. Starrykiller (/user/235125)
// I love Madoka Kaname forever!
#include <bits/stdc++.h>

using namespace std;

#ifndef __glibcxx_nonmember_container_access 
#define size(_Cont) _Cont.size()
#endif

constexpr int MAXN=2e3+10;

struct node {
    int t, a, b, c, d;
} op[MAXN]; int n;

signed main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin>>n;
    for (int i=1; i<=n; ++i) {
        string s; cin>>s;
        if (s=="POP") {
            op[i].t=1;
            cin>>op[i].a;
            cin>>s;
            cin>>op[i].b;
            cin>>s; 
            cin>>s;
            cin>>op[i].c;
            cin>>s;
            cin>>op[i].d;
        } else {
            op[i].t=2;
            cin>>s;
            cin>>op[i].a; cin>>s; 
            cin>>op[i].b;
        }
    }
    int cur=1; int cnt=0;
    bool flag=false;
    vector<int> stk;
    while (1) {
        int nxt=0;
        cnt++;
        if (op[cur].t==2) {
            if (!size(stk)) break;
            if (flag) {
                cnt=-1; break;
            }
            stk.push_back(op[cur].a);
            nxt=op[cur].b;
        } else {
            if (size(stk) && stk.back()==op[cur].a) {
                stk.pop_back();
                nxt=op[cur].b;
            } else {
                if (flag) {
                    cnt=-1; break;
                }
                stk.push_back(op[cur].c); nxt=op[cur].d;
            }
        }
        flag=nxt<=cur; cur=nxt;
    }
    cout<<cnt<<'\n';
}