n=int(input());x=0
for _ in range(n):
    a,b=input().split()
    h,m=map(int,a.split(':'));s,t=map(int,b.split(':'))
    c=0;c=(s+24*(s<h)-h)*60+t-m
    x+=c-1440*(c>1440)+1440*(c<0)
print(x)