#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> l_l;

#define EPS (1e-7)
#define INF (1e9)
#define PI (acos(-1))
//const ll mod = 1000000007;
ll N, M;
ll A[100050];
ll B[100050];
ll before[100050];
ll beforemax = 0;

int main() {
    //cout.precision(10);
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> N >> M;
    ll ans = 0;
    for(int i = 1; i <= N; i++) cin >> A[i] >> B[i];
    ll now = 0;
    for(int i = 1; i <= N; i++) {
        now += max(A[i], B[i]);
        ll delta = A[i] * (M - 1);
        if(A[i] < 0) {
            if(B[i] > A[i]) delta = 0;
            if(B[i] < A[i]) delta = max(delta, B[i] - A[i]);
        }
        before[i] = before[i - 1] + delta;
        beforemax = max(beforemax, before[i]);
        ans = max(ans, beforemax + now);
    }
    cout << ans << endl;
    return 0;
}