Buggy python submission (verdict: Runtime Error):
n,c=map(int,input().split())
x=[0]
v=[]
for i in range(n):
    a,b=map(int,input().split())
    x.append(a)
    v.append(b)

x1=[x[i+1]-x[i] for i in range(len(x)-1)]
x2=[c-x[-1]]+x1[:0:-1]
h=[0]
for i,j in zip(x2,reversed(v)):
    h.append(h[-1]+j+-i)
t=[0]
for i,j in zip(x1,v):
    t.append(t[-1]+j-i)
hi=h.index(max(h[0:-1]))
ti=t.index(max(t[0:-1]))
tm=max(t[0:-1])
hm=max(h[0:-ti])
h2=[h[i]-c+x[-i] for i in range(1,len(h)-1)]
t2=[t[i]-x[i] for i in range(1,len(t)-1)]


#print(ti,hi)
print(max(max(t), max(h), hm+max(t2[0:-ti]),tm+max(h2[:-hi])))

Fixed submission:
n,c=map(int, input().split())
a=[[0,0]]+[list(map(int, input().split())) for i in range(n)]+[[c,0]]
m=[0]
h=[0]
mb=[0]
hb=[0]
for i in range(n+1):
    m.append(m[-1]-a[i+1][0]+a[i][0]+a[i+1][1])
    mb.append(m[-2]-a[i+1][0]+a[i][0]+a[i+1][1]-a[i+1][0])
    h.append(h[-1]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1])
    hb.append(h[-2]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1]-c+a[-i-2][0])

mm=[0]
for i in range(n):
    mm.append(max(mm[-1],m[i+1]))
hm=[0]
for i in range(n):
    hm.append(max(hm[-1],h[i+1]))
    
ans=max(hm[-1],mm[-1])
for i in range(1,n):
    ans=max(hb[i]+mm[-i-1], mb[i]+hm[-i-1],ans)
print(ans)

Buggy python submission (verdict: Runtime Error):
n,c=map(int,input().split())
x=[0]
v=[]
for i in range(n):
    a,b=map(int,input().split())
    x.append(a)
    v.append(b)

x1=[x[i+1]-x[i] for i in range(len(x)-1)]
x2=[c-x[-1]]+x1[:0:-1]
h=[0]
for i,j in zip(x2,reversed(v)):
    h.append(h[-1]+j+-i)
t=[0]
for i,j in zip(x1,v):
    t.append(t[-1]+j-i)
hi=h.index(max(h[0:-1]))
ti=t.index(max(t[0:-1]))
tm=max(t[0:-1])
hm=max(h[0:-ti])
h2=[h[i]-c+x[-i] for i in range(1,len(h))]
t2=[t[i]-x[i] for i in range(1,len(t))]


#print(ti,hi)
print(max(max(t), max(h), hm+max(t2[0:-ti]),tm+max(h2[:-hi])))

Fixed submission:
n,c=map(int, input().split())
a=[[0,0]]+[list(map(int, input().split())) for i in range(n)]+[[c,0]]
m=[0]
h=[0]
mb=[0]
hb=[0]
for i in range(n+1):
    m.append(m[-1]-a[i+1][0]+a[i][0]+a[i+1][1])
    mb.append(m[-2]-a[i+1][0]+a[i][0]+a[i+1][1]-a[i+1][0])
    h.append(h[-1]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1])
    hb.append(h[-2]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1]-c+a[-i-2][0])

mm=[0]
for i in range(n):
    mm.append(max(mm[-1],m[i+1]))
hm=[0]
for i in range(n):
    hm.append(max(hm[-1],h[i+1]))
    
ans=max(hm[-1],mm[-1])
for i in range(1,n):
    ans=max(hb[i]+mm[-i-1], mb[i]+hm[-i-1],ans)
print(ans)

Buggy python submission (verdict: Runtime Error):
n,c=map(int,input().split())
x=[0]
v=[]
for i in range(n):
    a,b=map(int,input().split())
    x.append(a)
    v.append(b)

x1=[x[i+1]-x[i] for i in range(len(x)-1)]
x2=[c-x[-1]]+x1[:0:-1]
h=[0]
for i,j in zip(x2,reversed(v)):
    h.append(h[-1]+j+-i)
t=[0]
for i,j in zip(x1,v):
    t.append(t[-1]+j-i)
hi=h.index(max(h[0:-1]))
ti=t.index(max(t[0:-1]))
tm=max(t[0:-1])
hm=max(h[0:-ti])
h2=[h[i]-c+x[-i] for i in range(1,len(h))]
t2=[t[i]-x[i] for i in range(1,len(t))]
print(h,t)
print(h2,t2)

#print(ti,hi)
print(max(max(t), max(h), hm+max(t2[0:-ti]),tm+max(h2[:-hi])))

Fixed submission:
n,c=map(int, input().split())
a=[[0,0]]+[list(map(int, input().split())) for i in range(n)]+[[c,0]]
m=[0]
h=[0]
mb=[0]
hb=[0]
for i in range(n+1):
    m.append(m[-1]-a[i+1][0]+a[i][0]+a[i+1][1])
    mb.append(m[-2]-a[i+1][0]+a[i][0]+a[i+1][1]-a[i+1][0])
    h.append(h[-1]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1])
    hb.append(h[-2]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1]-c+a[-i-2][0])

mm=[0]
for i in range(n):
    mm.append(max(mm[-1],m[i+1]))
hm=[0]
for i in range(n):
    hm.append(max(hm[-1],h[i+1]))
    
ans=max(hm[-1],mm[-1])
for i in range(1,n):
    ans=max(hb[i]+mm[-i-1], mb[i]+hm[-i-1],ans)
print(ans)

Buggy python submission (verdict: Runtime Error):
n,c=map(int,input().split())
x=[0]
v=[]
for i in range(n):
    a,b=map(int,input().split())
    x.append(a)
    v.append(b)

x1=[x[i+1]-x[i] for i in range(len(x)-1)]
x2=[c-x[-1]]+x1[:0:-1]
h=[0]
for i,j in zip(x2,reversed(v)):
    h.append(h[-1]+j+-i)
t=[0]
for i,j in zip(x1,v):
    t.append(t[-1]+j-i)
hi=h.index(max(h[0:-1]))
ti=t.index(max(t[0:-1]))
tm=max(t[0:-1])
hm=max(h[0:-ti])
h2=[h[i]-c+x[-i] for i in range(1,len(h))]
t2=[t[i]-x[i] for i in range(1,len(h))]
print(h,t)
print(h2,t2)

#print(ti,hi)
print(max(max(t), max(h), hm+max(t2[0:-ti]),tm+max(h2[:-hi])))

Fixed submission:
n,c=map(int, input().split())
a=[[0,0]]+[list(map(int, input().split())) for i in range(n)]+[[c,0]]
m=[0]
h=[0]
mb=[0]
hb=[0]
for i in range(n+1):
    m.append(m[-1]-a[i+1][0]+a[i][0]+a[i+1][1])
    mb.append(m[-2]-a[i+1][0]+a[i][0]+a[i+1][1]-a[i+1][0])
    h.append(h[-1]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1])
    hb.append(h[-2]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1]-c+a[-i-2][0])

mm=[0]
for i in range(n):
    mm.append(max(mm[-1],m[i+1]))
hm=[0]
for i in range(n):
    hm.append(max(hm[-1],h[i+1]))
    
ans=max(hm[-1],mm[-1])
for i in range(1,n):
    ans=max(hb[i]+mm[-i-1], mb[i]+hm[-i-1],ans)
print(ans)

Buggy python submission (verdict: Runtime Error):
n,c=map(int,input().split())
x=[0]
v=[]
for i in range(n):
    a,b=map(int,input().split())
    x.append(a)
    v.append(b)

x1=[x[i+1]-x[i] for i in range(len(x)-1)]
x2=[c-x[-1]]+x1[:0:-1]
h=[0]
for i,j in zip(x2,reversed(v)):
    h.append(h[-1]+j+-i)
t=[0]
for i,j in zip(x1,v):
    t.append(t[-1]+j-i)
hi=h.index(max(h[0:-1]))
ti=t.index(max(t[0:-1]))
tm=max(t[0:-1])
hm=max(h[0:-ti])
h2=[h[i]-x[i] for i in range(1,len(h))]
t2=[t[i]-x[-i-1] for i in range(1,len(h))]
#print(h,t)
#print(h2,t2)

#print(ti,hi)
print(max(max(t), max(h), hm+max(t2[0:-ti]),tm+max(h2[:-hi])))
#print(hm, tm, hm+max(t2[0:-ti]),tm+max(h2[:-hi]))

Fixed submission:
n,c=map(int, input().split())
a=[[0,0]]+[list(map(int, input().split())) for i in range(n)]+[[c,0]]
m=[0]
h=[0]
mb=[0]
hb=[0]
for i in range(n+1):
    m.append(m[-1]-a[i+1][0]+a[i][0]+a[i+1][1])
    mb.append(m[-2]-a[i+1][0]+a[i][0]+a[i+1][1]-a[i+1][0])
    h.append(h[-1]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1])
    hb.append(h[-2]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1]-c+a[-i-2][0])

mm=[0]
for i in range(n):
    mm.append(max(mm[-1],m[i+1]))
hm=[0]
for i in range(n):
    hm.append(max(hm[-1],h[i+1]))
    
ans=max(hm[-1],mm[-1])
for i in range(1,n):
    ans=max(hb[i]+mm[-i-1], mb[i]+hm[-i-1],ans)
print(ans)

Buggy python submission (verdict: Runtime Error):
n,c=map(int,input().split())
x=[0]
v=[]
for i in range(n):
    a,b=map(int,input().split())
    x.append(a)
    v.append(b)
x1=[x[i+1]-x[i] for i in range(len(x)-1)]
x2=[c-x[-1]]+x1[:0:-1]
h=[0]
for i,j in zip(x2,reversed(v)):
    h.append(h[-1]+j+-i)
t=[0]
for i,j in zip(x1,v):
    t.append(t[-1]+j-i)
hi=h.index(max(h[0:-1]))
ti=t.index(max(t[0:-1]))
tm=max(t[0:-1])
hm=max(h[0:-ti])
h2=[h[i]-x[i] for i in range(len(h))]
t2=[t[i]-x[-i-1] for i in range(1,len(h))]
#print(h2,t2)

#print(ti,hi)
print(max( hm, tm, hm+max(t2[0:-ti]),tm+max(h2[:-ti])))


Fixed submission:
n,c=map(int, input().split())
a=[[0,0]]+[list(map(int, input().split())) for i in range(n)]+[[c,0]]
m=[0]
h=[0]
mb=[0]
hb=[0]
for i in range(n+1):
    m.append(m[-1]-a[i+1][0]+a[i][0]+a[i+1][1])
    mb.append(m[-2]-a[i+1][0]+a[i][0]+a[i+1][1]-a[i+1][0])
    h.append(h[-1]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1])
    hb.append(h[-2]-a[-i-1][0]+a[-i-2][0]+a[-i-2][1]-c+a[-i-2][0])

mm=[0]
for i in range(n):
    mm.append(max(mm[-1],m[i+1]))
hm=[0]
for i in range(n):
    hm.append(max(hm[-1],h[i+1]))
    
ans=max(hm[-1],mm[-1])
for i in range(1,n):
    ans=max(hb[i]+mm[-i-1], mb[i]+hm[-i-1],ans)
print(ans)