#include <iostream>

using namespace std;

typedef long long LL;

const int N = 110;

int n, d, b[N], t[N];
LL dp[N][2];

int main() {
    // freopen("work.in", "r", stdin);
    // freopen("work.out", "w", stdout);

    scanf("%d%d", &n, &d);
    for (int i = 1; i <= n; ++i) scanf("%d%d", &b[i], &t[i]);

    dp[1][0] = b[1];
    dp[1][1] = t[1] - d;
    for (int i = 2; i <= n; ++i) {
        dp[i][0] = max(dp[i - 1][0], dp[i - 1][1] - d) + b[i];
        dp[i][1] = max(dp[i - 1][1], dp[i - 1][0] - d) + t[i];
    }

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

    return 0;
}