CodeForces 670B Game of Robots

#pragma comment(linker, "/STACK:1024000000,1024000000")
#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<vector>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<iostream>
using namespace std;
typedef long long LL;
const double pi=acos(-1.0),eps=1e-6;
void File()
{
    freopen("D:\\in.txt","r",stdin);
    freopen("D:\\out.txt","w",stdout);
}
template <class T>
inline void read(T &x)
{
    char c=getchar(); x=0;
    while(!isdigit(c)) c=getchar();
    while(isdigit(c)) {x=x*10+c-‘0‘; c=getchar();}
}

int n,k;
int a[100010];

int main()
{
    scanf("%d%d",&n,&k);
    for(int i=1;i<=n;i++) scanf("%d",&a[i]);
    int L=1,R=n,pos;
    while(L<=R)
    {
        int mid=(L+R)/2;
        LL num=(LL)(1+mid)*(LL)mid/(LL)2;
        if(num>=(LL)k) R=mid-1,pos=mid;
        else L=mid+1;
    }

    k=k-(1+pos-1)*(pos-1)/2;
    printf("%d\n",a[k]);

    return 0;
}