#include<bits/stdc++.h>

namespace fast {
    // fast I/O by rsk0315 (update: 2019-10-14 16:56:15).
    // This version supports only integer inputs/outputs, single character
    // outputs, and string literal outputs.
    static size_t constexpr buf_size = 1 << 17;
    static size_t constexpr margin = 1;
    static char inbuf[buf_size + margin] = {};
    static char outbuf[buf_size + margin] = {};
    static __attribute__((aligned(8))) char minibuf[32];
    static size_t constexpr int_digits = 20;  // 18446744073709551615
    static uintmax_t constexpr digit_mask = 0x3030303030303030;
    static uintmax_t constexpr first_mask = 0x00FF00FF00FF00FF;
    static uintmax_t constexpr second_mask = 0x0000FFFF0000FFFF;
    static uintmax_t constexpr third_mask = 0x00000000FFFFFFFF;
    static uintmax_t constexpr tenpow[] = {
            1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000
    };
    template <typename Tp>
    using enable_if_integral = std::enable_if<std::is_integral<Tp>::value, Tp>;

    class scanner {
        char* pos = inbuf;
        char* endpos = inbuf + buf_size;

        void M_read_from_stdin() {
            endpos = inbuf + fread(pos, 1, buf_size, stdin);
        }
        void M_reread_from_stdin() {
            ptrdiff_t len = endpos - pos;
            if (!(inbuf + len <= pos)) return;
            memcpy(inbuf, pos, len);
            char* tmp = inbuf + len;
            endpos = tmp + fread(tmp, 1, buf_size-len, stdin);
            *endpos = 0;
            pos = inbuf;
        }

    public:
        scanner() { M_read_from_stdin(); }

        template <typename Integral,
                typename enable_if_integral<Integral>::type* = nullptr>
        void scan_parallel(Integral& x) {
            // See https://qiita.com/rsk0315_h4x/items/17a9cb12e0de5fd918f4
            if (__builtin_expect(endpos <= pos + int_digits, 0))
                M_reread_from_stdin();
            bool ends = false;
            typename std::make_unsigned<Integral>::type y = 0;
            bool neg = false;
            if (std::is_signed<Integral>::value && *pos == '-') {
                neg = true;
                ++pos;
            }
            do {
                memcpy(minibuf, pos, 8);
                long c = *(long*)minibuf;
                long d = (c & digit_mask) ^ digit_mask;
                int skip = 8;
                int shift = 8;
                if (d) {
                    int ctz = __builtin_ctzl(d);
                    if (ctz == 4) break;
                    c &= (1L << (ctz-5)) - 1;
                    int discarded = (68-ctz) / 8;
                    shift -= discarded;
                    c <<= discarded * 8;
                    skip -= discarded;
                    ends = true;
                }
                c |= digit_mask;
                c ^= digit_mask;
                c = ((c >> 8) + c*10) & first_mask;
                c = ((c >> 16) + c*100) & second_mask;
                c = ((c >> 32) + c*10000) & third_mask;
                y = y*tenpow[shift] + c;
                pos += skip;
            } while (!ends);
            x = (neg? -y: y);
            ++pos;
        }

        template <typename Integral,
                typename enable_if_integral<Integral>::type* = nullptr>
        void scan_serial(Integral& x) {
            if (__builtin_expect(endpos <= pos + int_digits, 0))
                M_reread_from_stdin();
            bool neg = false;
            if (std::is_signed<Integral>::value && *pos == '-') {
                neg = true;
                ++pos;
            }
            typename std::make_unsigned<Integral>::type y = *pos-'0';
            while (*++pos >= '0') y = 10*y + *pos-'0';
            x = (neg? -y: y);
            ++pos;
        }

        template <typename Integral,
                typename enable_if_integral<Integral>::type* = nullptr>
        // Use scan_parallel(x) only when x may be too large (about 10^12).
        // Otherwise, even when x <= 10^9, scan_serial(x) may be faster.
        void scan(Integral& x) { scan_serial(x); }

        void scan_serial(std::string& s) {
            // until first whitespace
            s = "";
            do {
                char* startpos = pos;
                while (*pos > ' ') ++pos;
                s += std::string(startpos, pos);
                if (*pos != 0) {
                    ++pos;  // skip the space
                    break;
                }
                M_reread_from_stdin();
            } while (true);
        }

        void scan(std::string& s) { scan_serial(s); }
    };

    class printer {
        char* pos = outbuf;

        void M_flush_stdout() {
            fwrite(outbuf, 1, pos-outbuf, stdout);
            pos = outbuf;
        }

    public:
        printer() = default;
        ~printer() { M_flush_stdout(); }

        void print(char c) {
            if (__builtin_expect(pos + 1 >= outbuf + buf_size, 0)) M_flush_stdout();
            *pos++ = c;
        }

        template <size_t N>
        void print(char const(&s)[N]) {
            if (__builtin_expect(pos + N >= outbuf + buf_size, 0)) M_flush_stdout();
            memcpy(pos, s, N-1);
            pos += N-1;
        }

        void print(char const* s) {
            while (*s != 0) {
                *pos++ = *s++;
                if (pos == outbuf + buf_size) M_flush_stdout();
            }
        }

        template <typename Integral,
                typename enable_if_integral<Integral>::type* = nullptr>
        void print(Integral x) {
            if (__builtin_expect(pos + int_digits >= outbuf + buf_size, 0))
                M_flush_stdout();
            if (x == 0) {
                *pos++ = '0';
                return;
            }
            if (x < 0) {
                *pos++ = '-';
                if (__builtin_expect(x == std::numeric_limits<Integral>::min(), 0)) {
                    switch (sizeof x) {
                        case 2: print("32768"); return;
                        case 4: print("2147483648"); return;
                        case 8: print("9223372036854775808"); return;
                    }
                }
                x = -x;
            }
            char* mp = minibuf + sizeof minibuf;
            *--mp = x % 10 + '0';
            size_t len = 1;
            typename std::make_unsigned<Integral>::type y = x / 10;
            while (y > 0) {
                *--mp = y % 10 + '0';
                y /= 10;
                ++len;
            }
            memcpy(pos, mp, len);
            pos += len;
        }

        template <typename Tp>
        void println(Tp const& x) { print(x), print('\n'); }
    };
} // fast::

// thank you ebi-chan!

fast::scanner ebin;
fast::printer ebout;

// fast IO

#warning "Newer version is available: /algorithm/radix_sort.cpp"

template <class RandomIt>
auto rsort(RandomIt first, RandomIt last)
-> typename std::enable_if<std::is_integral<
        typename RandomIt::value_type>::value>::type
{
    using IntType = typename RandomIt::value_type;
    intmax_t cur_shift=0, max_exp=sizeof(IntType);
    std::vector<IntType> work(last-first);
    for (int i=0; i<max_exp; ++i) {
        size_t num[256]={};

        for (RandomIt it=first; it<last; ++it)
            ++num[((*it)>>cur_shift)&255];

        {
            size_t tmp=0;
            for (int j=0; j<256; ++j)
                std::swap(tmp, num[j]+=tmp);
        }

        for (RandomIt it=first; it<last; ++it)
            work[num[((*it)>>cur_shift)&255]++] = *it;

        {
            size_t j=0;
            for (RandomIt it=first; it<last; ++it)
                *it = work[j++];
        }

        cur_shift += 8;
    }

    if (!std::is_signed<IntType>::value) return;

    RandomIt mid=last;
    for (RandomIt it=last; --it>=first;)
        if (*it >= 0) {
            mid = ++it;
            break;
        }

    if (mid == last)
        return;

    std::copy(first, mid, work.begin()+(last-mid));
    std::copy(mid, last, work.begin());
    std::copy(work.begin(), work.end(), first);
}

template <class RandomIt>
auto rsort(RandomIt first, RandomIt last)
-> typename std::enable_if<std::is_integral<
        typename RandomIt::value_type::first_type>::value>::type
{
    using PairType = typename RandomIt::value_type;
    using KeyType = typename PairType::first_type;

    intmax_t cur_shift=0, max_exp=sizeof(KeyType);
    std::vector<PairType> work(last-first);
    for (int i=0; i<max_exp; ++i) {
        size_t num[256]={};

        for (RandomIt it=first; it<last; ++it)
            ++num[((it->first)>>cur_shift)&255];

        {
            size_t tmp=0;
            for (int j=0; j<256; ++j)
                std::swap(tmp, num[j]+=tmp);
        }

        for (RandomIt it=first; it<last; ++it)
            work[num[((it->first)>>cur_shift)&255]++] = *it;

        {
            size_t j=0;
            for (RandomIt it=first; it<last; ++it)
                *it = work[j++];
        }

        cur_shift += 8;
    }

    if (!std::is_signed<KeyType>::value) return;

    RandomIt mid=last;
    for (RandomIt it=last; --it>=first;)
        if ((it->first) >= 0) {
            mid = ++it;
            break;
        }

    if (mid == last)
        return;

    std::copy(first, mid, work.begin()+(last-mid));
    std::copy(mid, last, work.begin());
    std::copy(work.begin(), work.end(), first);
}

// thank you ebi-chan!

int main(){
    using namespace std;
    unsigned long N, D, A;
    ebin.scan(N);
    ebin.scan(D);
    ebin.scan(A);
    vector<pair<unsigned long, unsigned long>> enemies(N);
    for(auto& i : enemies){
        ebin.scan(i.first);
        ebin.scan(i.second);
        (i.second += A - 1) /= A;
    }
    rsort(enemies.begin(), enemies.end());
    unsigned long ans{0}, now{0};
    queue<pair<unsigned long, unsigned long>> pq;
    for(const auto& i : enemies){
        while(!pq.empty() && pq.front().first < i.first && (now += pq.front().second))pq.pop();
        auto k = i.second - min(i.second, ans - now);
        if(k)pq.emplace(i.first + D * 2, k);
        ans += k;
    }
    cout << ans << endl;
    return 0;
}

Measured memory_bytes: 6656