#include <bits/stdc++.h>
using namespace std;
long long a, b, c, d, e, f, g, h, i, j, k, m, n;
void run(long long x, long long y, long long m) {
  long long count = 0;
  while (x < m && y < m) {
    if (x <= 0 && y <= 0) {
      count = -1;
      break;
    }
    if (x < 0) {
      if (m < 0)
        g = m;
      else
        g = 0;
      j = (g - x) / y;
      k = (g - x) % y;
      count += j + (k ? 0 : 1);
      x = k ? g : (g - k + y);
    } else if (y < 0) {
      if (m < 0)
        g = m;
      else
        g = 0;
      j = (g - y) / x;
      k = (g - y) % x;
      count += j + (k == 0 ? 0 : 1);
      y = k == 0 ? g : (g - k + x);
    } else if (x > y) {
      y = x + y;
      count++;
    } else if (y >= x) {
      x = x + y;
      count++;
    }
  }
  cout << count << endl;
}
int main() {
  cin >> a >> b >> c;
  run(a, b, c);
  return 0;
}
