#include <cstdlib>
#include <limits>
#include <numeric>
#include <vector>

long long tapeEquilibrium(const std::vector<int>& a)
{
    long long firstPart = 0;
    long long secondPart = std::accumulate(a.begin(), a.end(), 0LL);
    long long min = std::numeric_limits<long long>::max();

    for (std::size_t i = 0; i + 1 < a.size(); ++i) {
        // index `i` rather than `a[i]` here.
        firstPart += static_cast<long long>(i);
        secondPart -= static_cast<long long>(i);
        long long difference = std::llabs(firstPart - secondPart);
        min = std::min(min, difference);
    }

    return min;
}