#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;

ll dp[100005][5];

int main(){
  int N, M;
  cin >> N >> M;
  vector<vector<ll>> v;
  rep(i,N){
    ll A, B;
    cin >> A >> B;
    v.push_back({A,B});
  }

  REP(i,1,N+1){
    rep(j,3){
      dp[i][j] = -(ll)(1e16);
    }
  }
  
  rep(i,N){
    rep(j,3){
      ll base = dp[i][j];
      {//k==0
        dp[i+1][0] = max(dp[i+1][0], base);
      }
      {//k==1
        if(j>=1){
          dp[i+1][1] = max(dp[i+1][1], base + v[i][0]);
          dp[i+1][1] = max(dp[i+1][1], base + v[i][1]);
        }
      }
      {//k==M
        if(j>=2){
          dp[i+1][2] = max(dp[i+1][2], base + M * v[i][0]);
          dp[i+1][2] = max(dp[i+1][2], base + (M-1) * v[i][0] + v[i][1]);
          dp[i+1][2] = max(dp[i+1][2], base + v[i][1]);
        }
      }
    }
  }

  ll ret = max(max(dp[N][0], dp[N][1]), dp[N][2]);
  cout << ret << endl;
  
  return 0;
}