#include <algorithm>
#include <cassert>
#include <functional>
#include <iostream>
#include <queue>
#include <ranges>
#include <vector>

int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);

    int n;
    std::cin >> n;

    std::vector<std::pair<int, int>> lr(n);
    for (auto& [l, r] : lr) std::cin >> l >> r;

    std::ranges::sort(lr);

    int k = 0;
    for (int i = 0; i < n; ++i) {
        k = std::max(k, lr[i].first - i);
    }
    for (auto& [l, r] : lr) {
        l = std::max(l, k);
        if (l > r) {
            std::cout << -1 << std::endl;
            return 0;
        }
        l -= k, r -= k;
    }

    std::vector<std::vector<int>> bucket(n);
    for (int i = 0; i < n; ++i) {
        auto [l, r] = lr[i];
        bucket[l].push_back(r);
    }
    std::priority_queue<int, std::vector<int>, std::greater<int>> pq;
    for (int l = 0; l < n; ++l) {
        for (int r : bucket[l]) pq.push(r);
        assert(pq.size());
        if (pq.top() < l) {
            std::cout << -1 << std::endl;
            return 0;
        }
        pq.pop();
    }
    std::cout << k - 1 << std::endl;
}