function[sum_num] = num2(x, vl, spikepos)
x_p = repmat(x,length(spikepos),1);
d = vecnorm(x_p-spikepos, 2, 2);
D = 1.5*vl;
result = exp(-0.5*d(d<D)/vl);
sum_num = sum(result);