#include <bits/stdc++.h>
using namespace std;
int main() {
  long long cnt = 0;
  long long x, y, m;
  cin >> x >> y >> m;
  if (max(x, y) >= m) return cout << 0 << endl, 0;
  if (!x && !y) return cout << -1 << endl, 0;
  if (x <= 0 && y <= 0) return cout << -1 << endl, 0;
  if (x < 0) {
    long long x1 = (-1) * x;
    cnt = x1 / y;
    if (x1 % y) cnt++;
    x = x + cnt * y;
  } else if (y < 0) {
    long long y1 = (-1) * y;
    cnt = y1 / x;
    if (y1 % x) cnt++;
    y = y + cnt * x;
  }
  while (max(x, y) < m) {
    long long sum = x + y;
    if (x < y)
      x = sum;
    else
      y = sum;
    cnt++;
  }
  cout << cnt << endl;
  return 0;
}
