How to bound the curvature of a nilpotent Lie group?

Question

In "Almost flat manifolds" by Gromov, I found a statement (in sec 4.4) that If $|[x,y]|\leq c|x||y|$, $x,y\in \ell$, $c\geq 0$, then the curvature $c(L)$ satisfies $c(L)\leq 100c^2$, where $L$ is an n-dimensional simply connected nilpotent Lie group, and $\ell$ is its Lie algebra with Euclidean structure. Does anyone know how he found the coefficient "100"?