#include <bits/stdc++.h>
using namespace std;
long long x, y, m, ans, td;
int main() {
  cin >> x >> y >> m;
  if (x <= 0 && y <= 0) {
    if (x >= m || y >= m) {
      cout << 0;
      return 0;
    }
    cout << -1;
    return 0;
  }
  ans = 0;
  if (x >= m || y >= m) {
    cout << ans;
    return 0;
  }
  if ((x < 0 || y < 0) && (x > 0 || y > 0)) {
    if (x < 0) {
      td = -x;
      ans = td / y;
      x = x + ans * y;
    } else {
      td = -y;
      ans = td / x;
      y = y + ans * x;
    }
  }
  while (1) {
    ans++;
    if (x <= y)
      x = x + y;
    else
      y = x + y;
    if (x >= m || y >= m) break;
  }
  cout << ans;
}
