n=gets.to_i
a=[]
while cin=gets
    x,y=cin.split(" ").map(&:to_i)
    a+=[y-x > 0 ? y-x : -1]
    a.uniq!
    if a.size>1
        puts -1
        exit
    elsif a[0]==-1
        puts -1
        exit
    end
end
p a[0]