#include <algorithm>
#include <cmath>
#include <climits>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <cassert>
#include <functional>

using namespace std;

#define LOG(...) printf(__VA_ARGS__)
//#define LOG(...)
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define CLR(a) memset((a), 0 ,sizeof(a))

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

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

struct UnionFind {
	vector<int> v;
	UnionFind(int n) : v(n)  { for (int i = 0; i < n; i++) v[i] = i; }
	int  find(int x)         { return v[x] == x ? x : v[x] = find(v[x]); }
	void unite(int x, int y) { v[find(x)] = find(y); }
};
struct note{
	string date;
	int time;
	char e;
	int id;
};
int main() {
	int n;
	while (cin>>n,n) {
		vector<note> notes;
		vi bless(1000, 0);
		vi blessed(1000, -1);
		REP(i, n) {
			string s1, s2;
			char e;
			int id;
			cin >> s1 >> s2 >> e >> id;
			int time=atoi(s2.substr(0, 2).c_str()) * 60 + atoi(s2.substr(3, 2).c_str());
			if (id == 0) {
				if (e == 'I') {
					FOR(j,1, 1000)
						if (blessed[j] != -1)
							blessed[j] = time;
					blessed[0] = time;
				}
				else {
					FOR(j,1, 1000)
						if (blessed[j] != -1)
							bless[j] += time- blessed[j];
					blessed[0] = -1;
				}
			}
			else {
				if (e == 'I') {
					blessed[id] = time;
				}
				else {
					if(blessed[0]!=-1)
						bless[id] += time - blessed[id];
					blessed[id] = -1;
				}
			}
		}
		int ans = 0;
		FOR(i,1, bless.size())
			ans = max(ans, bless[i]);
		cout << ans << endl;
	}
}