#include <iostream>
#include <vector>

using namespace std;

int parse(string s) {
  int hh = stoi(s.substr(0, 2));
  int mm = stoi(s.substr(3, 2));
  int ss = stoi(s.substr(6, 2));
  return 3600 * hh + 60 * mm + ss;
}

int main() {
  int n, m;
  cin >> n >> m;
  vector<int> a(n), b(m);
  for(auto& x: a) {
    string s;
    cin >> s;
    x = parse(s);
  }
  for(auto& x: b) {
    string s;
    cin >> s;
    x = parse(s);
  }
  int thresh;
  cin >> thresh;
  int ret = 1e9;
  for(auto x: a) for(auto y: b) {
    if(y-x >= thresh) ret = min(ret, y - x);
  }
  if(ret == 1e9) ret = -1;
  cout << ret << "\n"; 
}