using namespaces

#include <cassert>
#include <algorithm>

namespace correct {
struct Foo {};

int global_variable = 0;

int64_t solve(int n) {
    int64_t result = 0;
    for (int i = std::min(0, n); i <= std::max(0, n); ++i) {
        result += i;
    }
    return result;
}
} //namespace correct


namespace fast {
struct Foo {};

int global_variable = 0;

int64_t solve(int n) {
    return 1ll * n * (n + 1) / 2;
}
} //namespace fast


int main() {
    for (int n : {0, 1, 2, 1'000'000}) { // C++11: 1000000
        assert(fast::solve(n) == correct::solve(n));
    }
    assert(fast::solve(-1) == correct::solve(-1)); // Oops!
}