import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=list(map(int,input().split()))

DP={abs(X[0]-(L-X[-1]))}

for i in range(1,N):
    NDP=set()

    for dp in DP:
        NDP.add(abs(dp+X[i]-X[i-1]))
        NDP.add(abs(dp-X[i]+X[i-1]))

    DP=NDP

print(min(DP))