Missing value in ordered vector problem

main.cpp

#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <cmath>

int main()
{
    std::vector<int> A = {1,3, 2,4,7,6};
    std::sort(A.begin(), A.end());
    
    std::vector<int> B(A.size());
    std::iota(B.begin(), B.end(), 1);
    
    std::vector<int> C;
    
    std::transform(A.begin(), A.end(), B.begin(), std::back_inserter(C),
    [](double a, double b) { return fabs(a-b); });
    
    
    
    std::cout << (std::find(C.begin(), C.end(), 1))-C.begin() << std::endl;
}