What value of gain k would lead to marginal stability?

In a closed loop system gain k= 10 and G1(s)= 1/(s^2 +13s+4)(s+1), I derived an expression for the closed loop transfer function h(s)= Y(s)/R(s) for gain k=10 to be = k (G1(s))/1 + k (G1(s)). What is the value of gain k which would lead to marginal stability?