#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9;
long long a, b, m;
int main() {
  cin >> a >> b >> m;
  if (a < m && b < m && a <= 0 && b <= 0) {
    cout << -1;
    return 0;
  }
  if (a >= m || b >= m) {
    cout << 0;
    return 0;
  }
  if (a > b) swap(a, b);
  long long c = 0;
  if (a < 0) {
    c = (b - a + b - 1ll) / b;
    a += b * c;
  }
  for (; a < m && b < m; c++) {
    if (a < b)
      a = a + b;
    else
      b = a + b;
  }
  cout << c;
  return 0;
}
