#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#define REP(i, n) for (int i = 0; i < (n); i++)

using namespace std;

long long dp0[100001];
long long dp1[100001];
long long dp2[100001];

void to(long long &x, long long y) {
  x = max(x, y);
}

int main() {
  int N, M;
  cin >> N >> M;
  vector<long long> A(N), B(N);
  REP(i, N) {
    cin >> A[i] >> B[i];
  }
  REP(i, N+1) dp0[i] = -1e18;
  REP(i, N+1) dp1[i] = -1e18;
  REP(i, N+1) dp2[i] = -1e18;
  dp0[0] = 0;
  REP(i, N) {
    to(dp0[i+1], dp0[i] + max({M*A[i], B[i], (M-1)*A[i]+B[i]}));
    if (i > 0) {
      to(dp1[i+1], dp0[i] + max(A[i], B[i]));
      to(dp2[i+1], dp0[i]);
    }
    to(dp1[i+1], dp1[i] + max(A[i], B[i]));
    to(dp2[i+1], dp1[i]);
    to(dp2[i+1], dp2[i]);
  }
  long long ans = max({dp0[N], dp1[N], dp2[N]});
  cout << ans << endl;
}