KeenOn Everything But Triangle

 #include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MAXN = (int)1e5+5;
struct node {
    int l,r,sum;
} T[MAXN*20];
int n,m,a[MAXN],root[MAXN],cnt,x,y,k,TT,q;
vector<int>v;
int getid(int x) {
    return lower_bound(v.begin(),v.end(),x)-v.begin()+1;
}
void updata(int l,int r,int &x,int y,int pos) {
    T[++cnt]=T[y],T[cnt].sum++,x=cnt;
    if(l==r)return;
    int mid=(l+r)/2;
    if(mid>=pos)updata(l,mid,T[x].l,T[y].l,pos);
    else updata(mid+1,r,T[x].r,T[y].r,pos);
}
int query(int l,int r,int x,int y,int k) {
    if(l==r)return l;
    int mid=(l+r)/2;
    int sum=T[T[y].l].sum-T[T[x].l].sum;
    if(sum>=k)return query(l,mid,T[x].l,T[y].l,k);
    else return query(mid+1,r,T[x].r,T[y].r,k-sum);
}
vector<ll>ans;
int main() {
    while(~scanf("%d%d",&n,&q)) {
        v.clear();
        cnt=0;
        for(int i=1; i<=n; i++)scanf("%d",&a[i]),v.push_back(a[i]),root[i]=0;
        sort(v.begin(),v.end()),v.erase(unique(v.begin(),v.end()),v.end());
        for(int i=1; i<=n; i++)updata(1,n,root[i],root[i-1],getid(a[i]));
        while(q--) {
            int l,r;
            scanf("%d%d",&l,&r);
            if(r-l+1<3) {
                printf("-1\n");
                continue;
            }
            bool flag=0;
            ll now=0;
            ll fi=(ll)v[query(1,n,root[l-1],root[r],r-l+1)-1];
            ll se=(ll)v[query(1,n,root[l-1],root[r],r-l)-1];
            for(int i=r-l-1; i>=1; i--) {
                ll th=(ll)v[query(1,n,root[l-1],root[r],i)-1];
                if(fi<se+th) {
                    flag=1;
                    now=fi+se+th;
                    break;
                }
                fi=se;
                se=th;
            }
            if(flag)printf("%lld\n",now);
            else printf("-1\n");
        }
    }
    return 0;
}