// LKM
// hw20.cpp
// Computes how many handshakes are needed if
// everyone in a room of N people shakes hands
// with everyone else.#include <iostream>

#include <iostream>
using namespace std;

int handshake(int n);

int main() {

    // Get input
    int N;
    cout << "Enter N: " << endl;
    cin  >> N;

    // Compute and show output
    cout << "For " << N << " people, " << handshake(N) 
         << " handshakes are needed." << endl;
    return 0;

}

// Computes how many handshakes for N people to each
// greet each other
int handshake(int n) {
    // base case -- one person means no shakes
    if (n <= 1) 
    {
        return 0;
    }

    // Recursive step -- shakes needed is number for N-1 people
    // plus shakes needed if one more person comes in (N-1)
    return handshake(n-1) + (n-1);
}