Binary search using vector pair

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll,ll>pll;
typedef pair<ll,pair<ll,ll> >pk;
#define sf(a) scanf("%d",&a)
#define pf(a) printf("%d\n",a)
#define F first
#define S second
#define minheap int,vector<int>,greater<int>
#define mp make_pair
#define pb push_back
#define pp pop_back
#define ischar(x)  (('a' <= x && x <= 'z') || ('A' <= x && x <= 'Z'))
#define fastread() (ios_base:: sync_with_stdio(false),cin.tie(NULL));
const int Max = 2e6 + 10;
const int Mod = 1e9 + 7;
bool compare(const pair<int,int> &a,
               const pair<int,int> &b)
{
       return (a.second > b.second);
}
ll b_search(vector<pll>ara,int low,int high,int k)
{
    while(low<=high)
    {
       int mid=(low+high)/2;
        if(ara[mid].F==k) return mid;
        if(k>ara[mid].F) low=mid+1;

        else high=mid-1;
    }
    return -1;
}

int main()
{
   fastread();


   vector<pll>v;
   int i,n,k;
   cin>>n;

   for(i=0; i<n; i++)
   {
       cin>>k;

       v.pb(mp(k,i));
   }

   sort(v.begin(),v.end());

   cin>>k;


   int sum=b_search(v,0,n-1,k);

   if(sum==-1)cout<<"Not found\n";

   else cout<<v[sum].S<<endl;

   cout<<endl;
}