const
    MAX_ID = 101010;

var
    a: array[1..MAX_ID] of integer;
    N, V: integer;
    f: text;

begin
    Assign(f, '27-122b.txt');
    reset(f);

    readln(f, N, V);

    for var i := 1 to MAX_ID do
        a[i] := 0;

    var L: int64 = 0;
    var R: int64 = 0;
    var sum: int64 = 0;

    for var i := 1 to N do
    begin
        var id, cnt: integer;
        readln(f, id, cnt);
        var x := (cnt + V - 1) div V;
        a[id] += x;
        R += x;
    end;

    for var id := 1 to MAX_ID do
        sum += id * a[id];

    var ans: int64 = sum;

    for var i := 1 to MAX_ID do
    begin
        sum -= R;
        sum += L;
        ans := min(ans, sum);
        R -= a[i];
        L += a[i];
    end;

    writeln(ans);
  
end.