import std;

void main() {
    int N, D;
    readf("%d %d\n", N, D);

    auto P = new long[](N), Q = new long[](N);
    foreach (i; 0 .. N) {
        readf("%d %d\n", P[i], Q[i]);
    }

    long res = long.min;
    foreach (i; 0 .. N) {
        foreach (j; 0 .. N) {
            if (i == j) {
                continue;
            }

            long diff = Q[i] + Q[j] - P[i] - P[j];
            if (diff >= 0) {
                long tmp = min(-P[i], -P[i]-P[j]+Q[i]);
                res = max(res, tmp);
            }
            else {
                long tmp = diff * ((D - 1) / 2);
                tmp -= P[i];
                if (D % 2 == 0) {
                    tmp = min(tmp, tmp-P[j]+Q[i]);
                }
                res = max(res, tmp);
            }
        }
    }

    res.writeln;
}