#!/usr/bin/env python3

import sys

n, m = [int(v) for v in sys.stdin.readline().split(' ')]

def get_time():
  time_str = sys.stdin.readline().strip()
  hh, mm, ss = map(int, time_str.split(':'))
  return 3600 * hh + 60 * mm + ss

arrivals = [get_time() for _ in range(n)]
departures = [get_time() for _ in range(m)]
s = int(sys.stdin.readline())

ans = -1
for i in range(n):
  for j in range(m):
    sol = departures[j] - arrivals[i]
    if sol >= s:
      ans = sol if ans == -1 else min(ans, sol)

print (ans)