#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000000000000

int main(){
	
	int N,M;
	cin>>N>>M;
	
	vector<long long> A(N),B(N);
	rep(i,N)scanf("%lld %lld",&A[i],&B[i]);
	
	vector dp(N,vector<long long>(2,-Inf));
	
	dp[0][0] = max({B[0],A[0]*M,A[0]*(M-1)+B[0]});
	
	for(int i=1;i<N;i++){
		dp[i][0] = dp[i-1][0] + max({B[i],A[i]*M,A[i]*(M-1)+B[i]});
		
		dp[i][1] = max(dp[i-1][0],dp[i-1][1]) + max(A[i],B[i]);
	}
	
	long long ans = -Inf;
	rep(i,N){
		rep(j,2){
			ans = max(ans,dp[i][j]);
		}
	}
	
	cout<<ans<<endl;
	
	return 0;
}