import sys
readline=sys.stdin.readline

N,D=map(int,readline().split())
dp0,dp1=0,-D
for i in range(N):
    T,K=map(int,readline().split())
    dp0,dp1=max(dp0+T,dp1+K-D),max(dp0+T-D,dp1+K)
ans=max(dp0,dp1)
print(ans)