Let \( c,k \in R \). If \( f(x)=(c+1)x^2+(1-c^2)x+2k \) and \( f(x+y)=f(x)+f(y)-xy \), for all \( x,y \in R \), then the value of \( \left| 2 \left( f(1)+f(2)+f(3)+\ldots+f(20) \right) \right| \) is equal to __________.