#include <bits/stdc++.h>
using namespace std;
void solve() {
  long long int x, y, m, t, c = 0;
  cin >> x >> y >> m;
  if (x > y) {
    t = x + y;
    x = t - x;
    y = t - x;
  }
  if (x <= 0 && y <= 0 && x < m && y < m) {
    cout << -1;
    return;
  }
  if (y >= m) {
    cout << 0;
    return;
  }
  if (x < 0 && y > 0) {
    t = abs(x) / y;
    x += t * y;
    c += t;
  }
  while (1) {
    if (x >= m || y >= m) {
      cout << c;
      return;
    }
    if (x < y)
      x = x + y;
    else
      y = y + x;
    c++;
  }
  return;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int t = 1;
  while (t--) {
    solve();
  }
  return 0;
}
