#include <bits/stdc++.h>
using namespace std;

#define rep(i,x,y) for(int i=(x);i<(y);++i)
#define debug(x) #x << "=" << (x)

#ifdef DEBUG
#define _GLIBCXX_DEBUG
#define print(x) std::cerr << debug(x) << " (L:" << __LINE__ << ")" << std::endl
#else
#define print(x)
#endif

const int inf=1e9;
const int64_t inf64=1e18;
const double eps=1e-9;

template <typename T> ostream &operator<<(ostream &os, const vector<T> &vec){
    os << "[";
    for (const auto &v : vec) {
    	os << v << ",";
    }
    os << "]";
    return os;
}

struct S{
    string date;
    int begin,end;
    
    S(){}
    
    S(string s,int b,int e)
    {
        date=s;
        begin=b;
        end=e;
    }
};

void solve(int n){
    vector<S> data[600];
    vector<int> ans(600,0);
    
    rep(i,0,n){
        string d;
        cin>>d;
        
        string temp;
        cin>>temp;
        
        int time=(temp[0]-'0')*600+(temp[1]-'0')*60+(temp[3]-'0')*10+(temp[4]-'0');
        
        string io;
        cin>>io;
        
        string sid;
        cin>>sid;
        
        int id=(sid[0]-'0')*100+(sid[1]-'0')*10+(sid[2]-'0');
        
        if(io=="I"){
            data[id].push_back(S(d,time,0));
        }else{
            data[id][data[id].size()-1].end=time;
        }
    }
    
    rep(i,1,500){
        rep(j,0,data[0].size()){
            rep(k,0,data[i].size()){
                if(data[0][j].date!=data[i][k].date){
                    continue;
                }
                
                int t_begin=max(data[0][j].begin,data[i][k].begin);
                int t_end=min(data[0][j].end,data[i][k].end);
                int t_ans=max(0,t_end-t_begin);
                
                ans[i]+=t_ans;
            }
        }
    }
    
    int answer=0;
    
    rep(i,0,500){
        answer=max(answer,ans[i]);
    }
    
    cout<<answer<<endl;
}

int main(){
    std::cin.tie(0);
    std::ios::sync_with_stdio(false);
    cout.setf(ios::fixed);
    cout.precision(10);
    
    while(true){
        int n;
        cin>>n;
        
        if(n==0) break;
        
        solve(n);
    }
    return 0;
}