Kirchoff's Current Law

The sum of currents flowing into the common point (a node) at any instant is just equal to the sum of the currents flowing out.

The arrows define the reference direction for positive current where current is defined by the motion of positive changes. The quantity i, specifies the magnitude of the current and its algebraic sign with respect to the reference direction. 

By Kirchoff's current law, the algebraic sum of the currents into a node Σi at any instant is zero.

Σi = 0 = i1 + i2 - i3 + i4