Consider two particles moving along the x-axis. In terms of their coordinates \( x_1 \) and \( x_2 \), their velocities are given as \( \frac{dx_1}{dt} = x_2 - x_1 \) and \( \frac{dx_2}{dt} = x_1 - x_2 \), respectively. When they start moving from their initial locations of \( x_1(0) = 1 \) and \( x_2(0) = -1 \), the time dependence of both \( x_1 \) and \( x_2 \) contains a term of the form \( e^{at} \), where \( a \) is a constant. The value of \( a \) (an integer) is: