Buggy python submission (verdict: Runtime Error):
import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=[0]+[int(input()) for i in range(N)]

def LENG(a,b,start):
    if b==0:
        if start>a:
            return a
        else:
            return L-a

    if a>b:
        a,b=b,a

    return a+L-b

MIN=1
MAX=N


ANS=0
while MAX-MIN>1:

    start=(MAX+MIN)//2
    ANSX=0
    if start==0 or start<0 or start>N:
        continue

    s=start
    t=(s+(-1)**0)%(N+1)
    ANS1=0


    
    for i in range(2,N+3):
        ANS1+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS1)
        
        s,t=t,(t-i*((-1)**i))%(N+1)

        

        if s==0:
            break
    #print(ANS1)

    s=start
    t=(s-(-1)**0)%(N+1)
    ANS2=0
    for i in range(2,N+3):
        ANS2+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS2)
        s,t=t,(t+i*((-1)**i))%(N+1)

        if s==0:
            break
    #print(ANS2)

    ANSX=max(ANS1,ANS2)
    
    start=(MAX+MIN)//2+1
    
    ANSY=0
    if start==0 or start<0 or start>N:
        continue

    s=start
    t=(s+(-1)**0)%(N+1)
    ANS1=0


    
    for i in range(2,N+3):
        ANS1+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS1)
        
        s,t=t,(t-i*((-1)**i))%(N+1)

        

        if s==0:
            break
    #print(ANS1)

    s=start
    t=(s-(-1)**0)%(N+1)
    ANS2=0
    for i in range(2,N+3):
        ANS2+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS2)
        s,t=t,(t+i*((-1)**i))%(N+1)

        if s==0:
            break
    #print(ANS2)

    ANSY=max(ANS1,ANS2)

    if ANSX>ANSY:
        MAX=N//2
    else:
        MIN=N//2+1

    #print(ANSX,ANSY)

print(max(ANSX,ANSY))
    


Fixed submission:
import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=[0]+[int(input()) for i in range(N)]

S=[]

for i in range(N):
    S.append(X[i+1]-X[i])
S.append(L-X[-1])

#print(S)

DIS=L-S[-1]
ANS=L-S[-1]
MINUS=0
PLUS=L-S[-1]

#print(DIS)

for i in range(N-1):
    if i%2==0:
        MINUS+=S[-i-2]
        PLUS+=S[-i//2-1]-S[-i-2]

    else:
        MINUS+=S[-i-2]-S[-i-1+i//2]
        PLUS-=S[-i-2]

    DIS+=PLUS-MINUS

    if ANS<DIS:
        ANS=DIS

    #print(DIS,PLUS,MINUS)

S=S[::-1]

DIS=L-S[-1]
MINUS=0
PLUS=L-S[-1]

#print(DIS)

for i in range(N-1):
    if i%2==0:
        MINUS+=S[-i-2]
        PLUS+=S[-i//2-1]-S[-i-2]

    else:
        MINUS+=S[-i-2]-S[-i-1+i//2]
        PLUS-=S[-i-2]

    DIS+=PLUS-MINUS

    if ANS<DIS:
        ANS=DIS

    #print(DIS,PLUS,MINUS)

print(ANS)

Buggy python submission (verdict: Runtime Error):
import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=[0]+[int(input()) for i in range(N)]

def LENG(a,b,start):
    if b==0:
        if start>a:
            return a
        else:
            return L-a

    if a>b:
        a,b=b,a

    return a+L-b

MIN=1
MAX=N


ANS=0
while MAX-MIN>1:

    start=N//2
    ANSX=0
    if start==0 or start<0 or start>N:
        continue

    s=start
    t=(s+(-1)**0)%(N+1)
    ANS1=0


    
    for i in range(2,N+3):
        ANS1+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS1)
        
        s,t=t,(t-i*((-1)**i))%(N+1)

        

        if s==0:
            break
    #print(ANS1)

    s=start
    t=(s-(-1)**0)%(N+1)
    ANS2=0
    for i in range(2,N+3):
        ANS2+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS2)
        s,t=t,(t+i*((-1)**i))%(N+1)

        if s==0:
            break
    #print(ANS2)

    ANSX=max(ANS,ANS1,ANS2)
    
    start=N//2+1
    
    ANSY=0
    if start==0 or start<0 or start>N:
        continue

    s=start
    t=(s+(-1)**0)%(N+1)
    ANS1=0


    
    for i in range(2,N+3):
        ANS1+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS1)
        
        s,t=t,(t-i*((-1)**i))%(N+1)

        

        if s==0:
            break
    #print(ANS1)

    s=start
    t=(s-(-1)**0)%(N+1)
    ANS2=0
    for i in range(2,N+3):
        ANS2+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS2)
        s,t=t,(t+i*((-1)**i))%(N+1)

        if s==0:
            break
    #print(ANS2)

    ANSY=max(ANS,ANS1,ANS2)

    if ANSX>ANSY:
        MAX=N//2
    else:
        MIN=N//2+1

print(max(ANSX,ANSY))
    


Fixed submission:
import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=[0]+[int(input()) for i in range(N)]

S=[]

for i in range(N):
    S.append(X[i+1]-X[i])
S.append(L-X[-1])

#print(S)

DIS=L-S[-1]
ANS=L-S[-1]
MINUS=0
PLUS=L-S[-1]

#print(DIS)

for i in range(N-1):
    if i%2==0:
        MINUS+=S[-i-2]
        PLUS+=S[-i//2-1]-S[-i-2]

    else:
        MINUS+=S[-i-2]-S[-i-1+i//2]
        PLUS-=S[-i-2]

    DIS+=PLUS-MINUS

    if ANS<DIS:
        ANS=DIS

    #print(DIS,PLUS,MINUS)

S=S[::-1]

DIS=L-S[-1]
MINUS=0
PLUS=L-S[-1]

#print(DIS)

for i in range(N-1):
    if i%2==0:
        MINUS+=S[-i-2]
        PLUS+=S[-i//2-1]-S[-i-2]

    else:
        MINUS+=S[-i-2]-S[-i-1+i//2]
        PLUS-=S[-i-2]

    DIS+=PLUS-MINUS

    if ANS<DIS:
        ANS=DIS

    #print(DIS,PLUS,MINUS)

print(ANS)

Buggy python submission (verdict: Time Limit Exceeded):
import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=[0]+[int(input()) for i in range(N)]

def LENG(a,b,start):
    if b==0:
        if start>a:
            return a
        else:
            return L-a

    if a>b:
        a,b=b,a

    return a+L-b


#CANDI=[(N+1)//2,(N+1)//2-1,(N+1)//2+1]
#(i+2)//2*(-1)**i

ANS=0
for start in range(1,N+1):
    if start==0:
        continue

    s=start
    t=(s+(-1)**0)%(N+1)
    ANS1=0


    
    for i in range(2,N+3):
        ANS1+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS1)
        
        s,t=t,(t-i*((-1)**i))%(N+1)

        

        if s==0:
            break
    #print(ANS1)

    s=start
    t=(s-(-1)**0)%(N+1)
    ANS2=0
    for i in range(2,N+3):
        ANS2+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS2)
        s,t=t,(t+i*((-1)**i))%(N+1)

        if s==0:
            break
    #print(ANS2)

    ANS=max(ANS,ANS1,ANS2)

print(ANS)
    


Fixed submission:
import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=[0]+[int(input()) for i in range(N)]

S=[]

for i in range(N):
    S.append(X[i+1]-X[i])
S.append(L-X[-1])

#print(S)

DIS=L-S[-1]
ANS=L-S[-1]
MINUS=0
PLUS=L-S[-1]

#print(DIS)

for i in range(N-1):
    if i%2==0:
        MINUS+=S[-i-2]
        PLUS+=S[-i//2-1]-S[-i-2]

    else:
        MINUS+=S[-i-2]-S[-i-1+i//2]
        PLUS-=S[-i-2]

    DIS+=PLUS-MINUS

    if ANS<DIS:
        ANS=DIS

    #print(DIS,PLUS,MINUS)

S=S[::-1]

DIS=L-S[-1]
MINUS=0
PLUS=L-S[-1]

#print(DIS)

for i in range(N-1):
    if i%2==0:
        MINUS+=S[-i-2]
        PLUS+=S[-i//2-1]-S[-i-2]

    else:
        MINUS+=S[-i-2]-S[-i-1+i//2]
        PLUS-=S[-i-2]

    DIS+=PLUS-MINUS

    if ANS<DIS:
        ANS=DIS

    #print(DIS,PLUS,MINUS)

print(ANS)

Buggy python submission (verdict: Runtime Error):
import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=[0]+[int(input()) for i in range(N)]

def LENG(a,b,start):
    if b==0:
        if start>a:
            return a
        else:
            return L-a

    if a>b:
        a,b=b,a

    return a+L-b


CANDI=[(N+1)//2,(N+1)//2-1,(N+1)//2+1]
#(i+2)//2*(-1)**i

ANS=0
for start in CANDI:
    if start==0:
        continue

    s=start
    t=(s+(-1)**0)%(N+1)
    ANS1=0


    
    for i in range(2,N+3):
        ANS1+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS1)
        
        s,t=t,(t-i*((-1)**i))%(N+1)

        

        if s==0:
            break
    #print(ANS1)

    s=start
    t=(s-(-1)**0)%(N+1)
    ANS2=0
    for i in range(2,N+3):
        ANS2+=LENG(X[s],X[t],X[start])
        #print(s,t,i,ANS2)
        s,t=t,(t+i*((-1)**i))%(N+1)

        if s==0:
            break
    #print(ANS2)

    ANS=max(ANS,ANS1,ANS2)

print(ANS)
    


Fixed submission:
import sys
input = sys.stdin.readline

L,N=map(int,input().split())
X=[0]+[int(input()) for i in range(N)]

S=[]

for i in range(N):
    S.append(X[i+1]-X[i])
S.append(L-X[-1])

#print(S)

DIS=L-S[-1]
ANS=L-S[-1]
MINUS=0
PLUS=L-S[-1]

#print(DIS)

for i in range(N-1):
    if i%2==0:
        MINUS+=S[-i-2]
        PLUS+=S[-i//2-1]-S[-i-2]

    else:
        MINUS+=S[-i-2]-S[-i-1+i//2]
        PLUS-=S[-i-2]

    DIS+=PLUS-MINUS

    if ANS<DIS:
        ANS=DIS

    #print(DIS,PLUS,MINUS)

S=S[::-1]

DIS=L-S[-1]
MINUS=0
PLUS=L-S[-1]

#print(DIS)

for i in range(N-1):
    if i%2==0:
        MINUS+=S[-i-2]
        PLUS+=S[-i//2-1]-S[-i-2]

    else:
        MINUS+=S[-i-2]-S[-i-1+i//2]
        PLUS-=S[-i-2]

    DIS+=PLUS-MINUS

    if ANS<DIS:
        ANS=DIS

    #print(DIS,PLUS,MINUS)

print(ANS)