// yukicoder: No.561 東京と京都
// 2019.5.12 bal4u

#include <stdio.h>

#if 0
#define gc() getchar_unlocked()
#else
#define gc() getchar()
#endif
int in()
{
	int n = 0, c = gc();
	do n = 10*n + (c & 0xf), c = gc(); while (c >= '0');
	return n;
}

int N, D;
int T[102], K[102];
int dp[102][2];

int main()
{
	int i, t;

	N = in(), D = in();
	for (i = 1; i <= N; i++) T[i] = in(), K[i] = in();
	dp[0][1] = -D;
	for (i = 1; i <= N; i++) {
		dp[i][0] = dp[i-1][0] + T[i];
		if ((t = dp[i-1][1] + K[i] - D) > dp[i][0])	dp[i][0] = t;
		dp[i][1] = dp[i-1][1] + K[i];
		if ((t = dp[i-1][0] + T[i] - D) > dp[i][1])	dp[i][1] = t;
	}
	t = dp[N][0];
	if (t < dp[N][1]) t = dp[N][1];
	printf("%d\n", t);
	return 0;
}