import math

R, n = input().split()
R, n = int(R), int(n)
rOld = sorted([int(i) for i in input().split(" ")])
if n == 1:
    res = 2*rOld[0]
else:
    r = []
    for i in range(len(rOld)):
        if i % 2 == 0:
            r.append(rOld[i])
        else:
            r = [rOld[i]] + r
    res = r[0] + r[-1]
    for i in range(n-1):
        res += math.sqrt((r[i]+r[i+1])**2 - (2*R-(r[i]+r[i+1]))**2)
print(round(res*1000))