task_1772

#include <iostream>
#include <set>

using namespace std;

void compare_and_change(set<pair<int, long long int>> & s, pair<int, long long int> & p);

int main() {
    int n, start, k, l, r;
    cin >> n >> start >> k;

    set<pair<int, long long int>> s;
    s.insert(pair<int, long long int>(start, 0));

    pair<int, long long int> left, right;
    long long int dl, dr;

    for (int i = 0; i < k; ++i) {
        cin >> l >> r;
        auto first = s.lower_bound(pair<int,long long int>(l, 0));
        if (first == s.end() || first->first > r)
            continue;

        left.first = l-1;
        left.second = 0;

        right.first = r+1;
        right.second = 0;

        auto it = first;
        while (it != s.end() && it->first <= r) {
            dl = it->second+it->first-l+1;
            dr = it->second+r-it->first+1;
            if (left.second == 0 || left.second > dl)
                left.second = dl;
            if (right.second == 0 || right.second > dr)
                right.second = dr;
            it++;
        }

        s.erase(first, it);

        if (l > 1) compare_and_change(s, left);
        if (r < n) compare_and_change(s, right);
    }

    long long int d = -1;
    for (auto & it : s)
        if (d == -1 || d > it.second)
            d = it.second;
    cout << d;
    return 0;
}

void compare_and_change(set<pair<int, long long int>> & s, pair<int, long long int> & p) {
    auto it = s.lower_bound(pair<int, long long int>(p.first, 0));
    if (it != s.end() && it->first == p.first) {
        if (it->second > p.second) {
            s.erase(it);
            s.insert(p);
        }
    } else s.insert(p);
}