Ok, are you sure that you are solving the right problem? I don't see you using the formula (x[i]-x[j])^2 + (y[i]-y[j])^2 anywhere...

Have you ever solved any minimum spanning tree problem before? If not, I recommend that you try this problem first: https://cses.fi/problemset/task/1675

For your problem, you only need to:

1. Create a graph G with N nodes.

2. Read X[U] and Y[U] for each node U from 0 to N — 1.

3. For each node U, for each other node V create an edge between U and V in G with a weight of square root of (X[U]-X[V])^2 + (Y[U]-Y[V])^2

4. Run Prim's algorithm

Furthermore, you need to do input/output from files. One (ugly) way to do it is to add this lines:

void solve(){
	ifstream cin("unionday.in");
	ofstream cout("unionday.out");	
	
	// your solution 
	
	cin.close();
	cout.close();
}