Let \( f(x,y) = e^{-x} \sin(-y), g(x,y) = e^{-x} cos(-y) \), and \( C \) be the square with vertices at \( (0,0), (\frac{\pi}{2},0), (\frac{\pi}{2},\frac{\pi}{2}), (0,\frac{\pi}{2}) \). Then, the value of the line integral \( \oint_C [f(x,y)dx + g(x,y)dy] \) is _______