https://ac.nowcoder.com/acm/contest/338/H

题解：排序，计算前缀和，模拟

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,q;
ll ans;
int a[N];
ll b[N];
char str;
int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    scanf("%d%d",&n,&k);
    for(int i=1;i<=n;i++)
        scanf("%d",&a[i]);
    sort(a+1,a+n+1,greater<int>());
    for(int i=1;i<=n;i++){
        b[i]=b[i-1]+a[i];
    }
    int l,r;
    for(int i=1;i<=k;i++){
        scanf("%d%d",&l,&r);
        ans=max(ans,b[r]-b[l-1]);
    }
    cout << ans << endl;
    //cout << "Hello world!" << endl;
    return 0;
}