CSES – Sum of Two Values

#include<bits/stdc++.h>
#define int long long
#define pii pair<int,int>
#define f first
#define s second
#define fastio ios_base::sync_with_stdio(0);cin.tie(0)
using namespace std;
//declare
const int maxn = 2e5+5;
int n,x;
vector<pii> v;
//
signed main(){
    fastio;
    cin>>n>>x;
    for(int i=1;i<=n;i++){
        int a; cin>>a;
        v.push_back({a,i});
    }
    if(n == 1){
        cout<<"IMPOSSIBLE\n";
        return 0;
    }
    sort(v.begin(),v.end());
    for(int i=0;i<v.size();i++){
        int k = x-v[i].f;
        pii p = {k,0};
        auto it = upper_bound(v.begin()+i+1, v.end(), p);
        if(it != v.end() && it->f == k){
            cout<<v[i].s<<" "<<it->s<<"\n";
            return 0;
        }
    }
    cout<<"IMPOSSIBLE\n";
    return 0;
}