/*
VIM: let g:lcppflags="-std=c++11 -O2 -pthread"
VIM: let g:wcppflags="/O2 /EHsc /DWIN32"
VIM: let g:argv=""
*/
#include <iostream>
#include <stdexcept>
#include <vector>
#include <algorithm>
#include <string>

int solution( std::vector<int> & a ) 
{
	long long s = 0;
	for ( auto it = a.begin(), ie=a.end(); it != ie; ++it ) {
		s += *it;
	}

	long long t = 0;
	for ( auto it = a.begin(), ie=a.end(); it != ie; ++it ) {
		if ( (s-*it)%2 == 0 && t == (s-*it)/2 )
			return it - a.begin();
		t += *it;
	}
	return -1;
}

int main ( void )
{try{
#if 1
	std::vector<int> a = {
		-1,3,-4,5,1,-6,2,1
	};

	std::cout << solution( a ) << std::endl;
	
#endif
	return 0;
}
catch ( const std::exception& e )
{
	std::cerr << std::endl
			<< "std::exception(\"" << e.what() << "\")." << std::endl;
	return 2;
}
catch ( ... )
{
	std::cerr  << std::endl
			<< "unknown exception." << std::endl;
	return 1;
}}

