#include <iostream>
#include <fstream>
#include <map>
#include <vector>
#include <string>
#include <algorithm>

using namespace std;

const int MAX_STATION = 1024;
const int MAX_TRAIN = 128;

int n, m;
int table[MAX_STATION];

typedef vector<pair<int, int> > Stops;

struct Train {
  Stops s;
  int i;
  bool run;
  int sleep;
  int sleep_max;

  bool prepare(int start) {
	for(i = 0; i < (int)s.size(); ++i)
	  if(s[i].second >= start)
		break;
	run = true;
	sleep = -1;
	sleep_max = 0;
	return i < (int)s.size();
  }
  bool next(int end) {
	if(run && sleep >= 0) {
	  sleep_max = max(sleep_max, sleep);
	  table[s[i].first] = max(table[s[i].first], sleep_max);
	} else if(!run) {
	  if(table[s[i].first] >= 0)
		sleep = max(sleep, 0);
	  if(sleep >= 0)
		sleep += s[i+1].second - s[i].second;
	  sleep_max = max(sleep_max, table[s[i].first]);
	  ++i;
	}
	run = !run;
	return (run || i+1 < (int)s.size()) && s[i].second <= end;
  }
  int getTime() const {
	return s[i].second * 2 + !run;
  }
};

Train train[MAX_TRAIN];

int Solve(int src_p, int src_t, int dst_p, int dst_t) {
  fill_n(table, n, -1);
  multimap<int, int> up;
  if(src_t > dst_t)
	return -1;
  table[src_p] = 0;
  for(int i = 0; i < m; ++i)
	if(train[i].prepare(src_t))
	  up.insert(make_pair(train[i].getTime(), i));
  while(!up.empty()) {
	int ti = up.begin()->second;
	up.erase(up.begin());
	if(train[ti].next(dst_t))
	  up.insert(make_pair(train[ti].getTime(), ti));
  }
  return table[dst_p];
}

int ReadTime(istream& s) {
  int h, m;
  char c;
  s >> h >> c >> m;
  return h*60 + m;
}

int ReadPos(istream& s) {
  int p;
  s >> p;
  --p;
  return p;
}

int main() {
  ifstream cin("D.txt");
  while(cin >> n >> m && n) {
	int sp = ReadPos(cin);
	int st = ReadTime(cin);
	int dp = ReadPos(cin);
	int dt = ReadTime(cin);
	for(int i = 0; i < m; ++i) {
	  int k;
	  cin >> k;
	  train[i].s.resize(k);
	  for(int j = 0; j < k; ++j) {
		train[i].s[j].first = ReadPos(cin);
		train[i].s[j].second = ReadTime(cin);
	  }
	}
	int ans = Solve(sp, st, dp, dt);
	if(ans >= 0)
	  cout << ans << endl;
	else
	  cout << "impossible" << endl;
  }
}