#include <iostream> using namespace std; long calc(long p, long pp) { return p + pp; } int main() { long a, n, m, x; cin >> a >> n >> m >> x; if (x <= 2) { cout << a << endl; return 0; } long o, pp = a, p = a; long tot = a; for (int i = 3; i < n; ++i) { long on = calc(p, pp); if (i < n - 1) { tot += on - p; pp = p; p = on; } else if (tot - p + on != m) { cout << "No." << endl; return 0; } } cout << tot << endl; return 0; }