In a $∆ABC$, the sides $BC, CA$ and $AB$ are consecutive positive integers in increasing order. Let $\overrightarrow{a},\overrightarrow{b}$ and $\overrightarrow{c}$ are position vectors of the vertices $A,B$ and $C$ respectively. If $\left(\overrightarrow{c}-\overrightarrow{a}\right)·\left(\overrightarrow{b}-\overrightarrow{c}\right)=0$, then the value of $\left|\overrightarrow{a}\times \overrightarrow{b}+\overrightarrow{b}\times \overrightarrow{c}+\overrightarrow{c}\times \overrightarrow{a}\right|$ is equal to