# Arup Guha
# 11/8/2025
# SER D2 Problem G: Shinjuku Station

toks = input().split()
n = int(toks[0])
m = int(toks[1])

times1 = []
for i in range(n):
    nums = [int(x) for x in input().split(":")]
    times1.append(nums[0]*3600+nums[1]*60+nums[2])

times2 = []
for i in range(m):
    nums = [int(x) for x in input().split(":")]
    times2.append(nums[0]*3600+nums[1]*60+nums[2])

mymin = int(input())

res = -1

# Normally we would sweep, but data is so small, let's
# just do the slow n*m
for x in times1:
    for y in times2:

        if y - x < mymin:
            continue

        if res == -1 or y - x < res:
            res = y - x

# Tada!
print(res)