A body of mass, $m$ is attached to the lower end of a spring whose upper end is fixed. The spring has negligible mass. When the mass $m$ is slightly pulled down and released, it oscillates with a time period of $3 \mathrm{s}$. When the mass $m$ is increased by $1 \mathrm{kg}$, the time period of oscillations becomes $5 \mathrm{s}$. The value of $m$ in $\mathrm{kg}$ is