Buggy python submission (verdict: Time Limit Exceeded):

import operator
class SegmentTree:
    def __init__(self, size, fn=operator.add, default=None, initial_values=None):
        """
        :param int size:
        :param callable fn: 区間に適用する関数。引数を 2 つ取る。min, max, operator.xor など
        :param default:
        :param list initial_values:
        """
        default = default or 0

        # size 以上である最小の 2 冪を size とする
        n = 1
        while n < size:
            n *= 2
        self._size = n
        self._fn = fn

        self._tree = [default] * (self._size * 2 - 1)
        if initial_values:
            i = self._size - 1
            for v in initial_values:
                self._tree[i] = v
                i += 1
            i = self._size - 2
            while i >= 0:
                self._tree[i] = self._fn(self._tree[i * 2 + 1], self._tree[i * 2 + 2])
                i -= 1

    def set(self, i, value):
        """
        i 番目に value を設定
        :param int i:
        :param value:
        :return:
        """
        x = self._size - 1 + i
        self._tree[x] = value

        while x > 0:
            x = (x - 1) // 2
            self._tree[x] = self._fn(self._tree[x * 2 + 1], self._tree[x * 2 + 2])

    def add(self, i, value):
        """
        もとの i 番目と value に fn を適用したものを i 番目に設定
        :param int i:
        :param value:
        :return:
        """
        x = self._size - 1 + i
        self.set(i, self._fn(self._tree[x], value))

    def get(self, from_i, to_i=None, k=0, L=None, r=None):
        """
        [from_i, to_i) に fn を適用した結果を返す
        :param int from_i:
        :param int to_i:
        :param int k: self._tree[k] が、[L, r) に fn を適用した結果を持つ
        :param int L:
        :param int r:
        :return:
        """
        if to_i is None:
            return self._tree[self._size - 1 + from_i]

        L = 0 if L is None else L
        r = self._size if r is None else r

        if from_i <= L and r <= to_i:
            return self._tree[k]

        if to_i <= L or r <= from_i:
            return None

        ret_L = self.get(from_i, to_i, k * 2 + 1, L, (L + r) // 2)
        ret_r = self.get(from_i, to_i, k * 2 + 2, (L + r) // 2, r)
        if ret_L is None:
            return ret_r
        if ret_r is None:
            return ret_L
        return self._fn(ret_L, ret_r)

    def __len__(self):
        return self._size



from bisect import bisect_right
def resolve():
    N, D, A = map(int, input().split())
    AB = [list(map(int, input().split())) for _ in range(N)]
    AB.sort()
    X, _ = zip(*AB)
    D = 2*D

    seg = SegmentTree(N+10)
    ans = 0
    for i, (x, h) in enumerate(AB):
        h = -(-h // A)
        damage = seg.get(0, i+1)
        
        if h < damage:
            continue
        ans += h - damage
        seg.add(i, h-damage)
        seg.add(bisect_right(X, x + D), - h + damage)
    print(ans)


if __name__ == "__main__":
    resolve()


Fixed submission:

def resolve():
    N, D, A = map(int, input().split())
    AB = [list(map(int, input().split())) for _ in range(N)]
    AB.sort()

    # imos
    imos = [0] * (N+1)
    ans = 0
    R = 0
    for l in range(N):
        if l:
            imos[l] += imos[l - 1]
        x = AB[l][0]
        h = AB[l][1]
        if imos[l] < h:
            R = max(R, l + 1)
            while R < N and AB[R][0] <= 2 * D + x:
                R += 1
            d = h - imos[l]
            cnt = (d+A-1) // A
            imos[l] += cnt * A
            imos[R] -= cnt * A
            ans += cnt
    print(ans)

if __name__ == "__main__":
    resolve()

Buggy python submission (verdict: Runtime Error):

def resolve():
    N, D, A = map(int, input().split())
    AB = [list(map(int, input().split())) for _ in range(N)]
    AB.sort()
    
    # imos
    x_max = AB[-1][0] + (2*D+5)
    imos = [0] * x_max
    ans = 0
    R = 0
    for l in range(N):
        if l:
            imos[l] += imos[l-1]
        x = AB[l][0]
        h = AB[l][1]
        if imos[l] < h:
            R = max(R, l+1)
            while R < N and AB[R][0] - x <= 2*D:
                R += 1
            d = h - imos[l]
            cnt = d//A
            imos[l] += cnt*A
            imos[R] -= cnt * A
            ans += cnt
    print(ans)

if __name__ == "__main__":
    resolve()


Fixed submission:

def resolve():
    N, D, A = map(int, input().split())
    AB = [list(map(int, input().split())) for _ in range(N)]
    AB.sort()

    # imos
    imos = [0] * (N+1)
    ans = 0
    R = 0
    for l in range(N):
        if l:
            imos[l] += imos[l - 1]
        x = AB[l][0]
        h = AB[l][1]
        if imos[l] < h:
            R = max(R, l + 1)
            while R < N and AB[R][0] <= 2 * D + x:
                R += 1
            d = h - imos[l]
            cnt = (d+A-1) // A
            imos[l] += cnt * A
            imos[R] -= cnt * A
            ans += cnt
    print(ans)

if __name__ == "__main__":
    resolve()