#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for (int (i)=(0);(i)<(int)(n);++(i))
using ll = long long;
using P = pair<ll, ll>;
using namespace std;

template<class T> void vin(vector<T>& v, int n) {
    v.resize(n);
    for (int i = 0; i < n; ++i) {
        cin >> v[i];
    }
}

ll N, M, A[101010], B[101010], dp[101010][3];

int main() {
    
    cin >> N >> M;
    for (int i=1; i<=N; ++i) cin >> A[i] >> B[i];

    dp[1][0] = -1e18;
    dp[1][1] = max(B[1],A[1]);
    dp[1][2] = max({A[1]*M, A[1]*(M-1)+B[1], B[1]});
    for (int i=2; i<=N; ++i) {
        dp[i][0] = max({dp[i-1][0], dp[i-1][1], dp[i-1][2]});
        dp[i][1] = max(dp[i-1][1], dp[i-1][2]) + max(A[i], B[i]);
        dp[i][2] = dp[i-1][2] + max({A[i]*M, A[i]*(M-1)+B[i], B[i]});
    }

    cout << max({dp[N][0], dp[N][1], dp[N][2]}) << endl;
}