없는 숫자 더하기

#include <string>
#include <vector>

using namespace std;

int solution(vector<int> numbers) 
{
    int answer = -1;
    int i, sum = 45, size = numbers.size();
    for(i = 0; i < size; i++)
    {
        sum = sum - numbers[i];
    }
    if(sum != 45)
    {
        answer = sum;
    }
    else
    {
        ;
    }
    
    return answer;
}