#include <bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) begin(v),end(v)
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
using ll = long long;
using pii = pair<int, int>;
constexpr ll INF = 1ll<<30;
constexpr ll longINF = 1ll<<60;
constexpr ll MOD = 1000000007;
constexpr bool debug = false;
//---------------------------------//

int main() {
	int N, D;
	cin >> N >> D;
	vector<int> T(N), K(N);
	REP(i, N) scanf("%d %d", &T[i], &K[i]);
	vector<int> dp { 0, -D }, ndp(2);
	REP(i, N) {
		ndp[0] = max(dp[0], dp[1] - D);
		ndp[1] = max(dp[0] - D, dp[1]);
		dp[0] = ndp[0] + T[i];
		dp[1] = ndp[1] + K[i];
	}
	printf("%d\n", max(dp[0], dp[1]));
}