#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int x, y, m, tmp, step, t1, t2;
  while (cin >> x >> y >> m) {
    if (x >= m || y >= m) {
      cout << 0 << endl;
      continue;
    }
    if (x > y) {
      tmp = x;
      x = y;
      y = tmp;
    }
    if (m >= 0) {
      if (x <= 0 && y <= 0) {
        cout << -1 << endl;
        continue;
      }
      step = 0;
      if (x < 0) {
        t1 = -x;
        t2 = (t1 / y) + (t1 % y != 0);
        step += t2;
        x += (t2 * y);
      }
      if (x > y) swap(x, y);
      while (y < m) {
        x = x + y;
        swap(x, y);
        step++;
      }
      cout << step << endl;
    } else if (m < 0) {
      if (x < m && y < m) {
        cout << -1 << endl;
        continue;
      } else {
        cout << 0 << endl;
        continue;
      }
    }
  }
  return 0;
}
