#include <iostream>
#include <set>
#include <map>
#include <algorithm>
 
int main()
{
	typedef std::map<int, int> M;
 
	M m { {1, -1}, {2, -2}, {3, -3}, {4, -4} };
 
	auto&& x = std::accumulate(std::begin(m), std::end(m), std::set<int>{},
	                           [](std::set<int>& s, const M::value_type& e) {//std::pair<const int, int>& e) {
	                           	  return s.insert(e.first), s;
	                           });
	for (auto& i : x)
	    std::cout << i << ' ';
	std::cout << '\n';
}

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