#include <iostream>
#include <cstring>

using namespace std;

typedef long long LL;

const int N = 110;

int n, d, a[N][2];
LL f[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", &a[i][0], &a[i][1]);

    memset(f, 0xcf, sizeof(f));
    f[0][0] = 0LL;
    for (int i = 1; i <= n; ++i) {
        for (int j = 0; j < 2; ++j) {
            f[i][j] = max(f[i - 1][j] + a[i][j], f[i - 1][1 - j] + a[i][j] - d);
        }
    }
    printf("%lld\n", max(f[n][0], f[n][1]));

    return 0;
}