5187: [Usaco2018 Jan]Sprinklers

#include<cstdio>
#include<iostream>
#include<cstring>
#include<cstdlib>
#include<algorithm>
#include<cmath>
using namespace std;
typedef long long LL;
const LL mod=1e9+;
int read()
{
    int x=,f=;char ch=getchar();
    while(ch<''||''<ch){if(ch=='-')f=-;ch=getchar();}
    while(''<=ch&&ch<=''){x=x*+ch-'';ch=getchar();}
    return x*f;
}

LL a[],up[],lw[];
LL l[],suml[],sumli[];
int main()
{
    int n,x,y;
    n=read();
    for(int i=;i<=n;i++)
        x=read(), y=read(), a[x]=y;

    //-----------------------------------------------------

    up[n-]=a[n-];for(int i=n-;i>=;i--)up[i]=max(up[i+],a[i]);
    lw[]=a[];    for(int i=;i<n;i++)   lw[i]=min(lw[i-],a[i]);

    int now=n;
    for(int i=;i<n;i++)
        while(now>lw[i])l[--now]=i;
    while(now>)l[--now]=n-;

    for(int i=n-;i>=;i--)
    {
        suml[i]=(suml[i+]+l[i])%mod;
        sumli[i]=(sumli[i+]+l[i]*(n-i)%mod)%mod;
    }

    //-----------------------------------------------------

    LL ans=,d;
    for(int i=;i<=n;i++)
    {
        d=(i*(up[i]-lw[i])*(up[i]+lw[i]+)/)%mod;
        ans=(ans+d)%mod;

        d=i*lw[i]%mod*(up[i]-lw[i])%mod;
        ans=((ans-d)%mod+mod)%mod;

        d=(sumli[lw[i]]-sumli[up[i]])%mod-(suml[lw[i]]-suml[up[i]])*(n-up[i])%mod;
        ans=((ans-d)%mod+mod)%mod;
    }
    printf("%lld\n",ans);

    return ;
}