#include "bits/stdc++.h"

#define REP(i,n) for(ll i=0;i<n;++i)
#define RREP(i,n) for(ll i=n-1;i>=0;--i)
#define FOR(i,m,n) for(ll i=m;i<n;++i)
#define RFOR(i,m,n) for(ll i=n-1;i>=m;--i)
#define ALL(v) (v).begin(),(v).end()
#define PB(a) push_back(a)
#define UNIQUE(v) v.erase(unique(ALL(v)),v.end());
#define DUMP(v) REP(aa, (v).size()) { cout << v[aa]; if (aa != v.size() - 1)cout << " "; else cout << endl; }
#define INF 1000000001ll
#define MOD 1000000007ll
#define EPS 1e-9

const int dx[8] = { 1,1,0,-1,-1,-1,0,1 };
const int dy[8] = { 0,1,1,1,0,-1,-1,-1 };


using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
ll max(ll a, int b) { return max(a, ll(b)); }
ll max(int a, ll b) { return max(ll(a), b); }
ll min(ll a, int b) { return min(a, ll(b)); }
ll min(int a, ll b) { return min(ll(a), b); }
///(?´????????`)(?´????????`)(?´????????`)(?´????????`)(?´????????`)(?´????????`)///

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);
	int n;
	while (cin >> n, n) {
		int flag = 0;
		vi v(1000, INF), ans(1000, 0);
		int t = 0;
		REP(i, n) {
			int M, D, h, m, p;
			char c;
			cin >> M >> c >> D >> h >> c >> m >> c >> p;
			if (c == 'I') {
				v[p] = h * 60 + m;
				if (p == 0) flag = 1;
			}
			else {
				if (flag) {
					if (p == 0) {
						FOR(i, 1, 1000) {
							ans[i] += max(0, h * 60 + m - max(v[i], v[0]));
						}
						flag = 0;
					}
					else ans[p] += max(0, h * 60 + m - max(v[p], v[0]));
				}
				v[p] = INF;

			}
		}
		int p, cmax = -1;
		FOR(i, 1, 1000) {
			if (ans[i] > cmax) {
				p = i;
				cmax = ans[i];
			}
		}
		cout << cmax << endl;
	}
}