#include <bits/stdc++.h>
using namespace std;

int main(){
    int64_t N, M, A[100000], B[100000];
    cin >> N >> M;
    for(int i=0; i<N; i++) cin >> A[i] >> B[i];

    int64_t a = 0, b = 0, c = 0;
    int64_t ans = -1e18;
    for(int i=0; i<N; i++){
        if(A[i] < 0 && B[i] < 0){
            a += (A[i] > B[i] ? A[i] : 0);
            b += max(A[i], B[i]);
            c += max(M*A[i], B[i]);
        }else{
            int64_t x, y;
            if(A[i] >= 0){
                x = A[i];
                y = max(A[i], B[i]);
            }else{
                x = 0;
                y = B[i];
            }
            a += x;
            b += y;
            c += (M-1)*x + y;
        }
        if(b <= c){
            a = 0;
            b = c;
        }
        if(i == 0){
            ans = c;
        }else{
            ans = max({ans, b, c});
        }
    }

    cout << ans << endl;
    return 0;
}