n=gets.to_i
q=[];
while cin=gets
    x,y=cin.split(" ").map(&:to_i)
    q+=[x+y*4];
end
t=q.max
p q.inject(0){|s,e|
    if (t-e)%2==1
        p -1;exit
    end
    s+(t-e)/2}