#include <iostream>
#include <algorithm>
#include <cstring>
#include <sstream>
#include <map>
#include <queue>
#include <vector>
#include <climits>
using namespace std;
typedef long long int ll;
typedef pair<ll, ll> pos;
const ll MOD = 1000000007,N=100010;
ll n, m, fn = INT_MIN,fn2=INT_MIN;
pos ab[N];
int main() {
	cin >> n >> m;
	for (ll i = 1,cfn=0; i <= n; i++) {
		cin >> ab[i].first >> ab[i].second;
		cfn += max(ab[i].first, ab[i].second);
		fn = max(cfn, fn);
	}
	for (ll i = 1, cfn2 = 0; i <= n; i++) {
		cfn2 += ab[i].first;
		fn2 = max(cfn2, fn2);
	}

	cout << fn + max((ll)0,fn2) * (m - 1) << endl;
	return 0;
}