#include <iostream>
#include <cstdio>

using namespace std;

typedef long long LL;
typedef pair<LL, LL> PLL;

const int N = 100010;
const LL NINF = -1e18;

PLL p[N];
int n, m;
LL dp[N][2], ans;

int main() {
    // freopen("dinner.in", "r", stdin);
    // freopen("dinner.out", "w", stdout);

    scanf("%d%d", &n, &m);
    for (int i = 1; i <= n; ++i) {
        scanf("%lld%lld", &p[i].first, &p[i].second);
    }

    dp[1][0] = max(max(p[1].first * m, p[1].second), p[1].first * (m - 1) + p[1].second);
    dp[1][1] = NINF;
    ans = dp[1][0];
    for (int i = 2; i <= n; ++i) {
        dp[i][0] = dp[i - 1][0] + max(max(p[i].first * m, p[i].second), p[i].first * (m - 1) + p[i].second);
        dp[i][1] = max(dp[i - 1][0], dp[i - 1][1]) + max(p[i].first, p[i].second);
        ans = max(max(ans, dp[i][0]), dp[i][1]);
    }

    printf("%lld\n", ans);

    return 0;
}