Three amigos

Margo and Sven are outside in a field, standing right next to each other and talking. We'll put x,y coordinates on the field and measure everything in meters. Ursula, a friend of both Margo and Sven, is also outside walking due north (positive y-direction) at $1~\mbox{m/s}$. When Ursula crosses the origin (at $t=0$) Margo and Sven (at coordinates $(100,0)$) notice her and decide to walk over and say hello.

Margo knows a little physics, and so she figures out how she should walk at $2~\mbox{m/s}$ to meet Ursula in the shortest amount of time. Sven doesn't know any physics, so he just always walks directly towards Ursula, also at $2~\mbox{m/s}$, until he catches up to her.

If the time Sven meets Ursula is $t_s$ and the time Margo meets Ursula is $t_m$, what is $t_s-t_m$ in seconds?