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#include<bits/stdc++.h>

const int W = 500001;
int64_t mx[W];

int main(){
	using namespace std;
	ios::sync_with_stdio(false), cin.tie(nullptr);
		
	int n, c;
	cin >> n >> c;
	
	vector<int> a(n + 1), w(n + 1);
	for(int i = 1;i <= n;i++){
		cin >> a[i] >> w[i];
	}
	
	vector<int64_t> dp(n + 1);
	int64_t max_dp = 0;
	int ptr = 1;
	for(int i = 1;i <= n;i++){
		while(ptr < i && a[ptr] < a[i]){
			max_dp = max(max_dp, dp[ptr]);
			mx[w[ptr]] = max(mx[w[ptr]], dp[ptr]);
			ptr += 1;
		}
		dp[i] = max(dp[i], a[i] + max_dp - c);
		dp[i] = max(dp[i], a[i] + mx[w[i]]);	
	}
	
	cout << *max_element(dp.begin(), dp.end());
	
	return 0;
}