/*
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 ) 
{
	auto n = a.size();

	int i = 0;
	for (int c = 0; c < n; ++c ) {
		auto new_i = i + a[i];
		if (new_i < 0 || n <= new_i)
			return i;
		i = new_i;
	}

	return -1;
}

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