Area under the graph

$\begin{aligned} I & = \int_0^{100} (x - {\color{#3D99F6}\lfloor x \rfloor})\ dx & \small \color{#3D99F6} \text{Note that }\lfloor x \rfloor = x - \{x\} \\ & = \int_0^{100} \{x\}\ dx & \small \color{#3D99F6} \text{where }0 \le \{x\} < 1 \text{ is the fraction part of }x. \\ & = 100 \int_0^1 x \ dx \\ & = 100 \cdot \frac {x^2}2 \bigg|_0^1 \\ & = \boxed{50} \end{aligned}$