BG-MO 2012 Problem 3

We are given a real number $a$ , not equal to $0$ or $1$ . Sacho and Deni play the following game. First is Sasho and then Deni and so on (they take turns). On each turn, a player changes one of the "*" symbols in the equation: $$*x^4+*x^3+*x^2+*x^1+*=0$$ with a number of the type $a^n$ , where $n$ is a whole number. Sasho wins if at the end the equation has no real roots, Deni wins otherwise. Determine (in term of $a$ ) who has a winning strategy