#include<bits/stdc++.h>
#define REP(i,n) for(int i=0;i<(n);i++)
#define ALL(v) (v).begin(),(v).end()
#define SZ(x) ((int)(x).size())
#define int long long
using namespace std;
typedef vector<int>   vint;
typedef pair<int,int> pint;

const int INF=1e18;

signed main()
{
    int N,M; cin>>N>>M;
    vint a(N),b(N),c(N);
    REP(i,N) cin>>a[i]>>b[i],c[i]=max(a[i],b[i]);

    vint sumA(N+1),sumB(N+1),sumC(N+1);
    REP(i,N){
        sumA[i+1]=sumA[i]+a[i];
        sumB[i+1]=sumB[i]+b[i];
        sumC[i+1]=sumC[i]+c[i];
    }

    int x=-INF,y=-INF;
    for(int i=1;i<=N;i++){
        x=max(x,sumA[i]);
        y=max(y,sumC[i]);
    }
    int ans=(M-1)*x+y;
    ans=max(ans,sumB[N]+(M-1)*max(x,0LL));
    cout<<ans<<endl;
}