#include <bits/stdc++.h>
using namespace std;

#define FOR(i,l,r) for(int i = int(l);i < int(r);i++)
template<typename T> bool chmax(T& a,const T& b){ return a < b ? (a = b,true) : false; }
template<typename T> bool chmin(T& a,const T& b){ return b < a ? (a = b,true) : false; }
typedef long long ll;

int N,D;
const int MAX_N = 100;
vector<int> T,K;
int dp [MAX_N + 1] [2];
const int INF = 1e9;

int main()
{
	scanf("%d%d",&N,&D);
	T.assign(N,0);
	K.assign(N,0);
	FOR(i,0,N){
		scanf("%d%d",&T [i],&K [i]);
	}

	fill(dp [0],dp [MAX_N + 1],-INF);
	dp [0] [0] = 0;
	FOR(i,0,N){
		chmax(dp [i + 1] [0],dp [i] [0] + T [i]);
		chmax(dp [i + 1] [1],dp [i] [0] + K [i] - D);
		chmax(dp [i + 1] [0],dp [i] [1] + T [i] - D);
		chmax(dp [i + 1] [1],dp [i] [1] + K [i]);
	}

	printf("%d\n",max(dp [N] [0],dp [N] [1]));

	return 0;
}