#include <iostream>
#include <vector>
#include <algorithm>

int INF = 987654321;
using ll = long long;
int n, d, t, k;

int main() {
    std::cin >> n >> d;
    int kyoto_price[n+1];
    int tokyo_price[n+1];
    int tokyo_dp[n];
    int kyoto_dp[n];
    kyoto_price[n] = {};
    tokyo_price[n] = {};
    tokyo_dp[n] = {};
    kyoto_dp[n] = {};
    kyoto_dp[0] = -INF;
    
    for (int i = 1; i < n; i++) {
        std::cin >> t >> k;
        tokyo_price[i] = t;
        kyoto_price[i] = k;
    }
    for (int i = 1; i <= n; i++) {
        tokyo_dp[i] = std::max(tokyo_dp[i-1], kyoto_dp[i-1]-d) + tokyo_price[i];
        kyoto_dp[i] = std::max(kyoto_dp[i-1], tokyo_dp[i-1]-d) + kyoto_price[i];
    }
    std::cout << std::max(tokyo_dp[n], kyoto_dp[n]) << std::endl;

}