#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];

int main() {
    //cout.precision(10);
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> N >> M;
    for(int i = 1; i <= N; i++) cin >> A[i] >> B[i];
    ll ans = 0;
    ll now = 0;
    for(int i = 1; i <= N; i++) {
        now += max(A[i], B[i]);
        ans = max(ans, now);
    }
    ll ans2 = 0;
    now = 0;
    for(int i = 1; i <= N; i++) {
        if(A[i] >= 0) {
            now += (M - 1) * A[i];
            ans2 = max(ans2, now);
            continue;
        }
        if(B[i] >= 0) {
            continue;
        }
        if(B[i] >= A[i]) {
            continue;
        }
        ll val = max((M - 1) * A[i], B[i] - A[i]);
        now += val;
    }
    cout << ans + ans2 << endl;
    return 0;
}