// #pragma GCC optimize ("Ofast")
// #pragma GCC optimize ("unroll-loops")
// #pragma GCC target ("avx,avx2,fma")
#include <bits/stdc++.h>

using std::cin, std::cout, std::cerr;
using ll = long long;

int main() {
    std::ios::sync_with_stdio(false);
    int n, d; cin >> n >> d;
    std::vector<int> f = {0, int(-1e9)};
    while(n --) {
        std::vector<int> g(2);
        int t, k; cin >> t >> k;
        g[0] = std::max(f[0], f[1] - d) + t;
        g[1] = std::max(f[1], f[0] - d) + k;
        f = g;
    }
    cout << std::max(f[0], f[1]) << '\n';
}