#include <bits/stdc++.h>
using namespace std;
const int OO = 2e9;
const int MOD = 1e9 + 7;
const int N = 1e5 + 5;
long long x, y, tmp, m, res = 0;
int main() {
  cin >> x >> y >> m;
  if ((x <= 0) && (y <= 0) && (x < m) && (y < m)) {
    cout << -1;
    return 0;
  }
  if (x > y) {
    tmp = x;
    x = y;
    y = tmp;
  }
  if (y >= m) {
    cout << 0;
    return 0;
  }
  if (x < 0) {
    res = (abs(x) / y);
    x = x + (abs(x) / y) * y;
  }
  if (x < 0) {
    x += y;
    res++;
  }
  while (y < m) {
    x = x + y;
    res++;
    if (x > y) {
      tmp = x;
      x = y;
      y = tmp;
    }
  }
  cout << res;
}
