#include <cstddef>
#include <vector>

int dominator(const std::vector<int>& a)
{
    int size = 0;
    int value = 0;
    int index = 0;

    for (std::size_t k = 0; k < a.size(); ++k) {
        int v = a[k];
        if (size == 0) {
            ++size;
            value = v;
            index = static_cast<int>(k);
        } else if (value != v) {
            --size;
        } else {
            ++size;
        }
    }

    int candidate = size > 0 ? value : -1;
    int count = 0;
    for (int v : a) {
        if (v == candidate) {
            ++count;
        }
    }
    if (count <= static_cast<int>(a.size()) / 2) {
        index = -1;
    }

    return index;
}