a, b = io.read("*n", "*n")
c = (3 - (a + b) % 3) % 3
mi, ma = 0, -1
if(c == 0) then
    if(a % 3 == 0) then
        mi = a; ma = b;
    elseif(a % 3 == 1) then
        mi = a + 2; ma = b - 2
    else
        mi = a + 1; ma = b - 1
    end
elseif(c == 1) then
    if(a % 3 == 0) then
        mi = a + 1; ma = b - 1;
    elseif(a % 3 == 1) then
        mi = a; ma = b
    else
        mi = a + 2; ma = b - 2
    end
else
    if(a % 3 == 0) then
        mi = a + 2; ma = b - 2;
    elseif(a % 3 == 1) then
        mi = a + 1; ma = b - 1
    else
        mi = a; ma = b
    end
end
io.write(string.format("%d\n", (ma - mi) / 3 + 1))