#include <iostream>

int sum_before(int n) {
    if (n < 0 || n > 100) {
        return -1;
    }
    int total{0};
    for (int i{0}; i < n; ++i) {
        total += i;
    }
    return total;
}

int main() {
    int n{4};
    int result{sum_before(n)};
    if (result < 0) {
        std::cout << "invalid" << '\n';
        return 1;
    }
    if (sum_before(0) != 0 || sum_before(1) != 0) {
        std::cout << "check failed" << '\n';
        return 1;
    }
    std::cout << result << '\n';
    return 0;
}