#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

typedef pair<int, int> Range;
typedef pair<int, int> FuelStop;

vector<Range> covered_ranges(const vector<FuelStop>& fuel_stops, const int full_distance, const int initial_fuel) {
    vector<Range> ranges{{0, initial_fuel}};
    ranges.reserve(fuel_stops.size());
    for (const auto& [distance, fuel_amount] : fuel_stops) {
        ranges.emplace_back(full_distance - distance, full_distance - distance + fuel_amount);
    }
    return ranges;
}

int min_stops(vector<Range> ranges, const int full_distance) {
    sort(ranges.begin(), ranges.end());
    int answer = 0;
    int current_pos = 0, max_r = -1;
    for (int i = 0; i < ranges.size();) {
        if (ranges[i].first <= current_pos) {
            max_r = max(max_r, ranges[i].second);
            if (max_r >= full_distance) { return answer - 1; }
            i++;
        } else {
            if (max_r < current_pos) { return -1; }
            answer++, current_pos = max_r, max_r = -1;
        }
    }
    return -1;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int fuel_stop_count, full_distance, initial_fuel;
    cin >> fuel_stop_count;
    vector<FuelStop> fuel_stops(fuel_stop_count);
    for (int i = 0; i < fuel_stop_count; i++) {
        cin >> fuel_stops[i].first >> fuel_stops[i].second;
    }
    cin >> full_distance >> initial_fuel;
    const vector<Range> ranges = covered_ranges(fuel_stops, full_distance, initial_fuel);
    for (const auto& [l, r] : ranges) {
        clog << "[" << l << "," << r << "] ";
    }
    clog << endl;

    cout << min_stops(ranges, full_distance) << endl;

    return 0;
}