#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
#define INF (1<<30)
#define INFLL (1ll<<60)
typedef pair<int, int> P;
typedef pair<int, P> E;
#define MOD (1000000007ll)
#define l_ength size

void mul_mod(ll& a, ll b){
	a *= b;
	a %= MOD;
}

void add_mod(ll& a, ll b){
	a += b;
	a %= MOD;
}

ll a[100100],b[100100],s[100100];

int main(void){
	int n,i;
	ll m,ans,p=0ll,q=0ll;
	cin >> n >> m;
	for(i=0; i<n; ++i){
		cin >> a[i] >> b[i];
	}
	for(i=(n-1); i>=0; --i){
		p += a[i];
		q += max(b[i]-a[i],0ll);
		if(p<0ll){
			q += p;
			q = max(0ll,q);
			p = 0ll;
		}
	}
	ans = max(0ll,p+q);
	ans = max(ans,p*m+q);
	cout << ans << endl;
	return 0;
}