#include <bits/stdc++.h>

using namespace std;

#define fst(t) std::get<0>(t)
#define snd(t) std::get<1>(t)
#define thd(t) std::get<2>(t)
#define unless(p) if(!(p))
#define until(p) while(!(p))

using ll = long long;
using P = std::tuple<int,int>;

const int dx[8] = {-1, 1, 0, 0, -1, -1, 1, 1}, dy[8] = {0, 0, -1, 1, -1, 1, -1, 1};

int N, D;
int T[110], K[110];
int dp[110][2];

int main(){
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    std::cin >> N >> D;

    for(int i=0;i<N;++i){
        std::cin >> T[i] >> K[i];
    }

    dp[0][0] = 0;
    dp[0][1] = -1001001001;

    for(int i=0;i<N;++i){
        dp[i+1][0] = max(dp[i][0], dp[i][1] - D) + T[i];
        dp[i+1][1] = max(dp[i][1], dp[i][0] - D) + K[i];
    }

    int res = max(dp[N][0], dp[N][1]);
    std::cout << res << std::endl;
}