#include <iostream>
#include <vector>
 
const int N = 10;
 
// Вычисляем сумму элементов от 1 до N, затем вычитаем каждый элемент in[]
// Результат - искомое число
int findMissingNumber(const int (&in)[N-1]) {
    long fullsum = 0;
    for (int i=0; i<N-1; ++i)
        fullsum += i+1-in[i];
    return fullsum+N;
}
 
const int in[] = {1, 6, 5, 3, 10, 4, 7, 9, 2};
 
int main(int argc, char** argv) {
    std::cout << findMissingNumber(in) << std::endl;
    return 0;
}

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