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#ifndef LOCAL
#pragma GCC optimize("O3,unroll-loops")
#endif
#include <bits/stdc++.h>
#define fi first
#define se second
#define pn printf("\n")
#define ssize(x) int(x.size())
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define bitcount(x) __builtin_popcount(x)
#define clz(x) __builtin_clz(x)
#define ctz(x) __builtin_ctz(x)
#define mp make_pair
//~ #define r(x) resize(x)
//~ #define rf(x, c) resize(x, c)
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<int, ll> pil;
typedef pair<ll, int> pli;
typedef pair<ll, ll> pll;
typedef double db;
typedef long double ldb;
#define V vector
int inf = 2e09; ll infll = 2e18; int mod = (1<<23)*119+1;
int add(int a, int b){return a+b >= mod ? a+b - mod : a+b;}
int sub(int a, int b){return a-b < 0 ? a-b + mod : a-b;}
int mul(int a, int b){return int(a * ll(b) % mod);}
int fpow(int a, ll b){
    int ret = 1;
    while(b){
        if(b & 1) ret = mul(ret, a);
        b >>= 1, a = mul(a, a);
    } return ret;
}
int inv(int a){ return fpow(a, mod-2); }
struct coeff{
    V<int> fac, invfac;
    coeff(int n){
        fac.resize(n+1), invfac.resize(n+1);
        fac[0] = 1, invfac[0] = 1;
        for(int i = 1; i <= n; ++i) fac[i] = mul(fac[i-1], i);
        invfac[n] = inv(fac[n]);
        for(int i = n-1; i; --i) invfac[i] = mul(invfac[i+1], i+1);
    }
    int get(int n, int k){
        if(n < k) return 0;
        return mul(fac[n], mul(invfac[n-k], invfac[k]));
    }
};
void answer(){
    int n; ll c; scanf("%d%lld", &n, &c);
    V<set<int>> t(500001);
    for(int i = 0, a, b; i < n; ++i)
        scanf("%d%d", &a, &b), t[a].emplace(b);
    V<ll> dp(500001, 0);
    ll best_dp = 0;
    for(int i = 500000; i >= 1; --i){
        ll new_best = 0;
        for(int u : t[i]){
            dp[u] = max(dp[u]+i, best_dp+i-c);
            new_best = max(new_best, dp[u]);
        }
        best_dp = max(best_dp, new_best);
    }
    printf("%lld\n", best_dp);
}
int main(){
		int T = 1;
		// scanf("%d", &T);
		//~ ios_base::sync_with_stdio(0); cin.tie(0); cin >> T;
		for(++T; --T; ) answer();
		return 0;
}