#include <bits/stdc++.h>
using namespace std;
static const int32_t INF = 0x3F3F3F3F;
typedef int64_t ll;
typedef vector<ll> vll;
ll N, D;
vll T, K;
ll dp[2][102];
struct Solution {
    void solve(std::istream& in, std::ostream& out) {
        in >> N >> D;
        memset(dp, 0, sizeof(dp));
        N++;
        T.resize(N);
        K.resize(N);
        dp[1][0] = -INF;
        for (int i = 0; i < N; i++) {
            in >> T[i] >> K[i];
            dp[0][i + 1] = max(T[i] + dp[0][i], T[i] + dp[1][i] - D);
            dp[1][i + 1] = max(K[i] + dp[0][i] - D, K[i] + dp[1][i]);
        }
        out << max(dp[0][N], dp[1][N]) << '\n';
    }
};
void solve(std::istream& in, std::ostream& out) {
    out << std::setprecision(12);
    Solution solution;
    solution.solve(in, out);
}
#include <fstream>
#include <iostream>
int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    istream& in = cin;
    ostream& out = cout;
    solve(in, out);
    return 0;
}