A company have $3$ factories, factory $1,$ factory $2,$ factory $3$ which make maple syrup. The company have to meet requirements of $4$ big supermarkets: IXAM, AGI, AP and ORTEM. The demand for each month is $7,12,17$ and $8$ tons for IXAM, AGI, AP and ORTEM respectively. The monthly capacity of factory $1$ and $2$ is $20$ and $15$ tons, respectively.

Knowing that this situation correspond to a minimum cost flow problem. What is the capacity of factory $3$ to make the problem feasible?