Challenge, and thus difficulty, is one of the main factors of enjoyment and motivation in video games. To enhance the players' motivation, many studies rely on Dynamic Difficulty Adjustment model in order to follow a difficulty curve. However, few authors worked on the shape of the difficulty curve itself. Our goal in this paper is to evaluate how players react to different difficulty curves. We use four different difficulty curves, including two flat curves and two curves with different baseline and peak levels. We test those curves on 67 students of a video games school while playing a First-Person Shooter game. Our study shows that curves with peaks have the strongest impact on players' motivation.