#include <bits/stdc++.h>
using namespace std;
template <class X, class Y>
void minimize(X &x, const Y &y) {
  if (x > y) {
    x = y;
  }
}
template <class X, class Y>
void maximize(X &x, const Y &y) {
  if (x < y) {
    x = y;
  }
}
long long x, y, m;
void init() { cin >> x >> y >> m; }
void solve() {
  if (x <= 0 && y <= 0 && m > 0) {
    cout << -1 << "\n";
    return;
  }
  if (max(x, y) >= m) {
    cout << 0 << "\n";
    return;
  }
  long long cou = 0, step;
  if (x < 0) {
    step = abs(x) / y;
    if (abs(x) % y) step++;
    x += y * step;
    cou = step;
  } else if (y < 0) {
    step = abs(y) / x;
    if (abs(y) % x) step++;
    y += x * step;
    cou = step;
  }
  long long tmp;
  while (max(x, y) < m) {
    tmp = x + y;
    if (x < y)
      x = x + y;
    else
      y = x + y;
    if (x + y < tmp) {
      cout << -1 << "\n";
      return;
    }
    cou++;
  }
  cout << cou << "\n";
}
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  init();
  solve();
}
