#include <iostream> #include <algorithm> #include <cstdio> using namespace std; typedef long long ll; ll dp[2][2]; ll n,d; int main(void){ scanf("%lld%lld",&n,&d); dp[0][1]=-20000000; for(int i=0;i<n;i++){ int u=(i+1)%2; ll a,b; scanf("%lld%lld",&a,&b); dp[u][0]=max(dp[i%2][0]+a,dp[i%2][1]+a-d); dp[u][1]=max(dp[i%2][0]+b-d,dp[i%2][1]+b); } printf("%lld\n",max(dp[n%2][0],dp[n%2][1])); }