given that f(x,y)= exp(-x^2+y^2/2), find probability of X+Y<1 when (X,Y) has this density.

my book said that dividing f(x,y) by a constant K will produce a bivariate pdf. hence the P(X+Y<1) would be a double integral over f(x,y) /K.

why do we need to divide by K and how do we know when to divide by K?