Ski-Trails for Robots

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <climits>
#include <cstring>
#include <string>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <vector>
#include <list>
#define rep(i,m,n) for(i=m;i<=n;i++)
#define rsp(it,s) for(set<int>::iterator it=s.begin();it!=s.end();it++)
#define mod 1000000007
#define inf 0x3f3f3f3f
#define vi vector<int>
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define ll long long
#define pi acos(-1.0)
#define pii pair<int,int>
#define Lson L, mid, rt<<1
#define Rson mid+1, R, rt<<1|1
const int maxn=1e5+10;
const int dis[4][2]={{0,1},{-1,0},{0,-1},{1,0}};
using namespace std;
ll gcd(ll p,ll q){return q==0?p:gcd(q,p%q);}
ll qpow(ll p,ll q){ll f=1;while(q){if(q&1)f=f*p%mod;p=p*p%mod;q>>=1;}return f;}
int n,m,k,t,s;
set<int>p,q;
set<int>::iterator now,pr,la;
ll dp[maxn];
int main()
{
    int i,j;
    scanf("%d%d%d",&n,&s,&k);
    rep(i,0,n+1)dp[i]=1e18;
    p.insert(0),p.insert(n+1),p.insert(s);
    dp[s]=0;
    while(k--)
    {
        int a,b;
        scanf("%d%d",&a,&b);
        if(a>1)
        {
            a--;
            p.insert(a);
            now=p.find(a);
            pr=--now;
            ++now;
            la=++now;
            --now;
            if(dp[*now]>dp[*pr]+(*now)-(*pr))dp[*now]=dp[*pr]+(*now)-(*pr);
            if(dp[*now]>dp[*la]+(*la)-(*now))dp[*now]=dp[*la]+(*la)-(*now);
            a++;
        }
        if(b<n)
        {
            b++;
            p.insert(b);
            now=p.find(b);
            pr=--now;
            ++now;
            la=++now;
            --now;
            if(dp[*now]>dp[*pr]+(*now)-(*pr))dp[*now]=dp[*pr]+(*now)-(*pr);
            if(dp[*now]>dp[*la]+(*la)-(*now))dp[*now]=dp[*la]+(*la)-(*now);
            b--;
        }
        q.clear();
        for(now=p.lower_bound(a);now!=p.end()&&*now<=b;now++)q.insert(*now);
        for(int x:q)p.erase(x),dp[x]=1e18;
    }
    ll mi=1e18;
    rep(i,1,n)if(mi>dp[i])mi=dp[i];
    printf("%lld\n",mi);
    //system("pause");
    return 0;
}