Directed Graphs and Network Flows · Medium problem

A network has arcs $s\to a$ (capacity $6$), $s\to b$ (capacity $4$), $a\to b$ (capacity $2$), $a\to t$ (capacity $5$), $b\to t$ (capacity $5$). Someone proposes the assignment $f(s,a)=6$, $f(s,b)=4$, $f(a,b)=2$, $f(a,t)=3$, $f(b,t)=5$. Every arc respects its capacity, but this is NOT a valid flow. By how many units does inflow exceed outflow at vertex $a$?