local ffi = require 'ffi'
local C, ct = ffi.C, ffi.typeof("int32_t[1]")
ffi.cdef 'int scanf(const char *, ...);'

print((function (n)
	local a,p,q,m,t,r={},ct(),ct(),0,0,0
	C.scanf("%d %d",p,q) t=p[0]+4*q[0] a[1],m=t,t
	for i=2,n do
		C.scanf("%d %d",p,q) t=p[0]+4*q[0] a[i]=t if t>m then m=t end
	end
	for i=1,#a do t=m-a[i] if t%2~=0 then return -1 end r=r+t/2 end return r
end)(io.stdin:read("*n")))