a, b, c, d = io.read("*n", "*n", "*n", "*n")
if(b < c) then
    print((b - a + 1) * (d - c + 1))
elseif(a <= c) then
    if(b <= d) then
        print((c - a) * (d - c + 1) + (b - c + 1) * (d - c))
    else
        print((d - c + 1) * (b - a))
    end
elseif(b < d) then
    print((b - a + 1) * (d - c))
elseif(a <= d) then
    print((d - a + 1) * (d - c) + (b - d) * (d - c + 1))
else
    print((b - a + 1) * (d - c + 1))
end