n=read_line.to_i
k=0
n.times{
    a,b=read_line.split
    h1,m1=a.split(":").map(&.to_i)
    h2,m2=b.split(":").map(&.to_i)
    p=h2*60+m2-h1*60-m1
    p+=1440 if p<0
    k+=p
}
puts k