Buggy python submission (verdict: Time Limit Exceeded):
import bisect


class SegTree:

    def __init__(self, N):
        self.N = N
        self.data = [0] * (2 * N)

    def update(self, i, x):
        i += self.N - 1
        self.data[i] = x
        while i > 0:
            i = (i - 1) // 2
            v = self.data[2 * i + 1] + self.data[2 * i + 2]
            self.data[i] = v

    def query(self, a, b, k, l, r):
        if r <= a or b <= l:
            return 0
        if a <= l and r <= b:
            return self.data[k]
        else:
            v = self.query(a, b, 2 * k + 1, l, (l + r) // 2)
            w = self.query(a, b, 2 * k + 2, (l + r) // 2, r)
            return v + w


def main():
    N, D, A = list(map(int, input().split(' ')))
    pw2_N = 1
    while pw2_N < N:
        pw2_N *= 2
    monsters = [list(map(int, input().split(' '))) for _ in range(N)]
    monsters.sort()
    if N == 1:
        print((monsters[0][1] + A - 1) // A)
        return
    counts = [(h + A - 1) // A for _, h in monsters]
    X = [x for x, _ in monsters]
    diffs = [(counts[i] - counts[i - 1]) if i > 0 else counts[i] for i in range(N)]
    tree = SegTree(pw2_N)
    for i, d in enumerate(diffs):
        tree.update(i, d)
    ans = 0
    for i in range(N):
        ci = tree.query(0, i + 1, 0, 0, pw2_N)
        if ci <= 0:
            continue
        ans += ci
        r = X[i] + 2 * D
        j = bisect.bisect_right(X, r)
        di = tree.query(i, i + 1, 0, 0, pw2_N)
        dj = tree.query(j, j + 1, 0, 0, pw2_N)
        tree.update(i, di - ci)
        tree.update(j, dj + ci)
    print(ans)


if __name__ == '__main__':
    main()

Fixed submission:
import bisect


def main():
    N, D, A = list(map(int, input().split()))
    monsters = [list(map(int, input().split())) for _ in range(N)]
    monsters.sort()
    X = [m[0] for m in monsters]
    # 端から貪欲に攻撃していく
    ans = 0
    damages = [0] * (N + 1)
    for n, monster in enumerate(monsters):
        x, h = monster
        h = max(0, h - damages[n])
        to_n = bisect.bisect_right(X, x + 2 * D)
        cnt = (h + A - 1) // A  # ceil(h / A)
        ans += cnt
        damages[n] += A * cnt
        damages[to_n] -= A * cnt
        damages[n + 1] += damages[n]
    print(ans)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
import sys
import bisect


sys.setrecursionlimit(10 ** 6)


class SegTree:

    def __init__(self, N):
        self.N = N
        self.data = [0] * (2 * N)

    def update(self, i, x):
        i += self.N - 1
        self.data[i] = x
        while i > 0:
            i = (i - 1) // 2
            v = self.data[2 * i + 1] + self.data[2 * i + 2]
            self.data[i] = v

    def query(self, a, b, k, l, r):
        if r <= a or b <= l:
            return 0
        if a <= l and r <= b:
            return self.data[k]
        else:
            v = self.query(a, b, 2 * k + 1, l, (l + r) // 2)
            w = self.query(a, b, 2 * k + 2, (l + r) // 2, r)
            return v + w


def main():
    N, D, A = list(map(int, input().split(' ')))
    pw2_N = 1
    while pw2_N < N:
        pw2_N *= 2
    monsters = [list(map(int, input().split(' '))) for _ in range(N)]
    monsters.sort()
    counts = [(h + A - 1) // A for _, h in monsters]
    X = [x for x, _ in monsters]
    diffs = [(counts[i] - counts[i - 1]) if i > 0 else counts[i] for i in range(N)]
    tree = SegTree(pw2_N)
    for i, d in enumerate(diffs):
        tree.update(i, d)
    ans = 0
    for i in range(N):
        ci = tree.query(0, i + 1, 0, 0, pw2_N)
        if ci <= 0:
            continue
        ans += ci
        r = X[i] + 2 * D
        j = bisect.bisect_right(X, r)
        di = tree.query(i, i + 1, 0, 0, pw2_N)
        dj = tree.query(j, j + 1, 0, 0, pw2_N)
        tree.update(i, di - ci)
        tree.update(j, dj + ci)
    print(ans)


if __name__ == '__main__':
    main()

Fixed submission:
import bisect


def main():
    N, D, A = list(map(int, input().split()))
    monsters = [list(map(int, input().split())) for _ in range(N)]
    monsters.sort()
    X = [m[0] for m in monsters]
    # 端から貪欲に攻撃していく
    ans = 0
    damages = [0] * (N + 1)
    for n, monster in enumerate(monsters):
        x, h = monster
        h = max(0, h - damages[n])
        to_n = bisect.bisect_right(X, x + 2 * D)
        cnt = (h + A - 1) // A  # ceil(h / A)
        ans += cnt
        damages[n] += A * cnt
        damages[to_n] -= A * cnt
        damages[n + 1] += damages[n]
    print(ans)


if __name__ == '__main__':
    main()

Buggy python submission (verdict: Runtime Error):
import bisect


class SegTree:

    def __init__(self, N):
        self.N = N
        self.data = [0] * (2 * N)

    def update(self, i, x):
        i += self.N - 1
        self.data[i] = x
        while i > 0:
            i = (i - 1) // 2
            v = self.data[2 * i + 1] + self.data[2 * i + 2]
            self.data[i] = v

    def query(self, a, b, k, l, r):
        if r <= a or b <= l:
            return 0
        if a <= l and r <= b:
            return self.data[k]
        else:
            v = self.query(a, b, 2 * k + 1, l, (l + r) // 2)
            w = self.query(a, b, 2 * k + 2, (l + r) // 2, r)
            return v + w


def main():
    N, D, A = list(map(int, input().split(' ')))
    pw2_N = 1
    while pw2_N < N:
        pw2_N *= 2
    monsters = [list(map(int, input().split(' '))) for _ in range(N)]
    monsters.sort()
    counts = [(h + A - 1) // A for _, h in monsters]
    X = [x for x, _ in monsters]
    diffs = [(counts[i] - counts[i - 1]) if i > 0 else counts[i] for i in range(N)]
    tree = SegTree(pw2_N)
    for i, d in enumerate(diffs):
        tree.update(i, d)
    ans = 0
    for i in range(N):
        ci = tree.query(0, i + 1, 0, 0, pw2_N)
        if ci <= 0:
            continue
        ans += ci
        r = X[i] + 2 * D
        j = bisect.bisect_right(X, r)
        di = tree.query(i, i + 1, 0, 0, pw2_N)
        dj = tree.query(j, j + 1, 0, 0, pw2_N)
        tree.update(i, di - ci)
        tree.update(j, dj + ci)
    print(ans)


if __name__ == '__main__':
    main()

Fixed submission:
import bisect


def main():
    N, D, A = list(map(int, input().split()))
    monsters = [list(map(int, input().split())) for _ in range(N)]
    monsters.sort()
    X = [m[0] for m in monsters]
    # 端から貪欲に攻撃していく
    ans = 0
    damages = [0] * (N + 1)
    for n, monster in enumerate(monsters):
        x, h = monster
        h = max(0, h - damages[n])
        to_n = bisect.bisect_right(X, x + 2 * D)
        cnt = (h + A - 1) // A  # ceil(h / A)
        ans += cnt
        damages[n] += A * cnt
        damages[to_n] -= A * cnt
        damages[n + 1] += damages[n]
    print(ans)


if __name__ == '__main__':
    main()