n, x, y = io.read("*n", "*n", "*n")
ref = y - x
if(ref <= 0) then ref = -1
else
    
    for i = 1, n - 1 do
        x, y = io.read("*n", "*n")
        if(ref ~= y - x) then
            ref = -1
            break
        end
    end
end
print(ref)