n, d = [ int(v) for v in input().split() ]
salalyinTokyo, salalyinKyoto = [ int(v) for v in input().split() ]
WorkinTokyomax, WorkinKyotomax = salalyinTokyo, salalyinKyoto - d

for i in range(1,n):
    salalyinTokyo, salalyinKyoto = [ int(v) for v in input().split() ]
    WorkinTokyomax, WorkinKyotomax = max(WorkinTokyomax + salalyinTokyo, WorkinKyotomax + salalyinTokyo - d), max(WorkinTokyomax + salalyinKyoto - d, WorkinKyotomax + salalyinKyoto)


print(max(WorkinTokyomax, WorkinKyotomax))