Question 12
Each cell of a $2020\times2020$ table is filled with a number which is either $1$ or $-1$. For $i=1,\ldots,2020$, let $R_i$ be the product of all the numbers in the $i$th row and let $C_i$ be the product of all the numbers in the $i$th column. Suppose $R_i+C_i=0$ for all $i=1,\ldots,2020$. What is the least number of $-1$'s in the table?