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#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cstdio>
#include <climits>
#include <vector>
#include <cmath>
#include <string>
#include <algorithm>
#include <string>
#include <deque>
#include <iomanip>

#define F first
#define S second

using namespace std;

const long double eps2 = 0.0000005;
const long double eps1 = 1e-5;
const long double pi = 3.1415926535897932;
const long long inf = INT_MAX / 2;

vector <pair <long long, long long> > otr;
pair <long long, long long> dp[100010];
//long long tree[400004];
vector<long long> tree;

void query(long long v, long long l, long long r, long long ql, long long qr, long long i)
{
    if (l == ql && r == qr)
    {
        tree[v] = i;
        return;
    }

    long long m = (l + r) / 2;

    if (qr <= m)
        return query(2 * v + 1, l, m, ql, qr, i);

    if (ql >= m)
        return query(2 * v + 2, m, r, ql, qr, i);

    query(2 * v + 1, l, m, ql, m, i);
    query(2 * v + 2, m, r, m, qr, i);

}

long long findprep(long long v, long long l, long long r, long long i)
{
    if (l + 1 == r)
        return tree[v];

    long long m = (l + r) / 2;

    long long ans;
    if (i < m)
        ans = findprep(2 * v + 1, l, m, i);
    else
        ans = findprep(2 * v + 2, m, r, i);

    return min(ans, tree[v]);
}

int main()
{
    ios_base::sync_with_stdio(0);
    //freopen("input.txt", "r", stdin);
    //      freopen("output.txt", "w", stdout);

    long long n, s, k;
    cin >> n >> s >> k;
    s--;
    s--;

    tree.resize(400004);
    tree.assign(400004, inf);
    otr.push_back(make_pair(0, s));
    long long l, r;
    for (long long i = 0; i < k; i++)
    {
        cin >> l >> r;
        l--; r--;
        otr.push_back(make_pair(l, r));

    }

    for (long long i = 0; i < 100010; i++)
        dp[i] = make_pair(0, 0);
    long long v1, v2;
    bool f1, f2;
    for (long long i = k; i >= 0; i--)
    {
        f1 = 1;
        f2 = 1;
        long long x = otr[i].F;
        long long y = otr[i].S;
        if (x == 0)
        {
            dp[i].F = inf;
            f1 = 0;
        }
        if (y == n - 1)
        {
            dp[i].S = inf;
            f2 = 0;
        }
        if (f1)
            v1 = findprep(0, 0, n + 1, x - 1);
        if (f2)
            v2 = findprep(0, 0, n + 1, y + 1);

        if (f1)
        {
            if (v1 == inf)
            {
                dp[i].F = 0;
            }
            else
            {
                dp[i].F = min(dp[v1].F + x - otr[v1].F, dp[v1].S + (otr[v1].S + 1 - (x - 1)));
            }
        }

        if (f2)
        {
            if (v2 == inf)
            {
                dp[i].S = 0;
            }
            else
            {
                dp[i].S = min(dp[v2].F + y + 1 - (otr[v2].F - 1), dp[v2].S + otr[v2].S - y);
            }
        }
        if (x <= y)
            query(0, 0, n + 1, x, y + 1, i);
    }

    cout << dp[0].S << endl;

    return 0;
}