f = 0;
[t, phi] = ode45(@f1, 0:0.0001:20, [0.5, 0]);
for i = 2:size(t)
    if ~f && phi(i, 2) > phi(1, 2)
        f = 1;
    elseif f && phi(i, 2) < phi(1, 2)
        t(i)
        break
    end
end