why map's key can't be inserted into a vector by iterator?

Question

I want to get map's keys, save in a vector, so i write the following code:

#include <vector>
#include <iostream>
#include <map>

using namespace std;

int main() {
  map<int, int> m = {{1,2}, {4,5}, {6,7}};
  vector<int> a(m.begin()->first, m.end()->first);
  for (auto i : a) {
    cout << i << endl;   // output 3 here
  }
}

this code print 3 only, which confuse me. I never input 3 into map, where is 3 comes out?