Školjka

%V0 The variables $a,b,c,d,$ traverse, independently from each other, the set of positive real values. What are the values which the expression $$S= \frac{a}{a+b+d} + \frac{b}{a+b+c} + \frac{c}{b+c+d} + \frac{d}{a+c+d}$$ takes?