n = io.read("*n", "*l")
res = 0
for i = 1, n do
    s = io.read()
    a, b, c, d = string.match(s, "(%d+):(%d+) (%d+):(%d+)")
    a, b, c, d = tonumber(a), tonumber(b), tonumber(c), tonumber(d)
    if(c < a or (c == a and d < b)) then c = c + 24 end
    res = res + (c - a) * 60 + d - b
end
print(res)