#include <bits/stdc++.h>
using namespace std;
struct Fast {
    Fast() {
        std::cin.tie(0);
        ios::sync_with_stdio(false);
        cout.precision(20);
    }
} fast;

/* define */
#define FOR(I, X, Y) for(long long(I) = (X); (I) < (Y); (I)++)
#define REP(I, X, Y) for(long long(I) = (Y)-1; (I) >= (X); (I)--)
#define ALL(X) (X).begin(), (X).end()
#define pb push_back
#define COUNT(V, X)                                                            \
    (upper_bound((V).begin(), (V).end(), X) -                                  \
     lower_bound((V).begin(), (V).end(), X))
#define debug(x) cerr << #x << ':' << x << endl;
#define DEBUG(v)                                                               \
    {                                                                          \
        cerr << #v << ':';                                                     \
        for(auto xv : v) cerr << xv << ' ';                                    \
        cerr << endl;                                                          \
    }
#define Yes(X) cout << (X ? "Yes" : "No") << endl;
#define YES(X) cout << (X ? "YES" : "NO") << endl;
#define ctoi(C) (C - '0')
#define pow2(x) ((long long)((long long)1 << x))

/* alias */
using ll = long long;
using ld = long double;
using vi = vector<int>;
using vii = vector<vector<int>>;
using vl = vector<long long>;
using vll = vector<vector<long long>>;
using pi = pair<int, int>;
using pl = pair<long long, long long>;
template <typename T> using PQ = priority_queue<T>;
template <typename T> using minPQ = priority_queue<T, vector<T>, greater<T>>;

/* const */
const long long dx[] = {1, 0, -1, 0};
const long long dy[] = {0, 1, 0, -1};
const long long dx8[] = {1, 1, 0, -1, -1, -1, 0, 1};
const long long dy8[] = {0, 1, 1, 1, 0, -1, -1, -1};
const long long dx9[] = {1, 1, 0, -1, -1, -1, 0, 1, 0};
const long long dy9[] = {0, 1, 1, 1, 0, -1, -1, -1, 0};
const int INF = 1000000007;
const long long LINF = 1000000000000000007;

/* func */
template <typename T1, typename T2> inline bool chmin(T1 &a, const T2 &b) {
    if(a > b) a = b;
    return a > b;
}
template <typename T1, typename T2> inline bool chmax(T1 &a, const T2 &b) {
    if(a < b) a = b;
    return a < b;
}
long long max(long long x, int y) {
    return max(x, (long long)y);
}
long long max(int x, long long y) {
    return max((long long)x, y);
}
long long min(long long x, int y) {
    return min(x, (long long)y);
}
long long min(int x, long long y) {
    return min((long long)x, y);
}

/* liblary */

/* main */

signed main() {
    ll n;
    while(true) {
        cin >> n;
        if(n == 0) break;
        set<string> s;
        map<string, vector<pair<string, pl>>> v;
        map<string, ll> tmp;
        FOR(i, 0, n) {
            string md, hm, e, p;
            cin >> md >> hm >> e >> p;
            ll h = stoi(hm.substr(0, 2));
            ll m = stoi(hm.substr(3, 2));
            ll t = m + h * 60;
            s.insert(md);
            if(e == "I") tmp[p] = t;
            if(e == "O") v[md].push_back(make_pair(p, make_pair(tmp[p], t)));
        }
        map<string, ll> sum;
        set<string> people;
        for(auto md : s) {
            vector<pl> gods;
            for(auto x : v[md]) {
                if(x.first == "000") {
                    gods.pb(x.second);
                }
            }
            for(auto god : gods) {
                for(auto x : v[md]) {
                    auto time = x.second;
                    ll t;
                    if(time.second <= god.first || god.second <= time.first)
                        continue;
                    if(god.first <= time.first) {
                        t = min(god.second, time.second) - time.first;
                    } else {
                        t = min(god.second, time.second) - god.first;
                    }
                    if(x.first != "000") {
                        people.insert(x.first);
                        sum[x.first] += t;
                    }
                }
            }
        }
        ll ans = 0;
        for(auto p : people) {
            ans = max(ans, sum[p]);
        }
        cout << ans << endl;
    }
}


Measured memory_bytes: 3288