working soln1

upen1.cpp

#include <iostream>
#include <vector>
#include <cassert>

using namespace std;

int main(){
	
	int N;
	cin >> N;
	const int CN=N;
	vector<int> v1(N);

	while(N--){
		cin >> v1[CN-N-1];
	}

	assert(CN >= 2);

	int sign = -1;
	int sum = 0;
	for (int i = 1; i < CN-1; ++i){
		sum += sign * 2 * v1[i];
		sign *= -1;
	}

	sum = sign * (sum + v1[0] + sign*v1[CN-1]);

	float rad;
	if(!(CN & 1)){
		// cout << "odd " <<sum << endl;
		rad = sum / (float)3;
	}

	if(rad > 0 && rad < v1[CN-1]-v1[0] ){
 		cout << rad << endl;
		return 0;
	}

	cout << "no soln\n";

	return 0;
}