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#include<iostream>

#include<vector>

#include<algorithm>

#include<map>

using namespace std;

int countPairs(int k, vector<int> a)

{

unordered_map<int, int> m;

vecter<int>::iterator it;

int n = a.size();

for (int it = a.begin(); it! = a.end(); it++)

m[*it]+=1;

int t = 0;

for (int it = a.begin(); it! = a.end(); it++){

t += m[ k- (*it) ];

if (k - (*it) == (*it)) t--;

}

return t/2;

}

int main(){

a[7] = {6,5,1,2,3,4,5}

vector<int> v(a,a+6)

cout << countPairs(7,v)<<endl;

return 0;

}