#include <bits/stdc++.h>
using namespace std;
long long x, y, m, cnt = 0;
long long solve(long long a, long long b) {
  long long ret = 0;
  if (a >= m || b >= m) return 0;
  if (a >= b)
    ret = solve(a, a + b) + 1;
  else if (a < b)
    ret = solve(a + b, b) + 1;
  return ret;
}
int main() {
  cin >> x >> y >> m;
  if (x <= 0 && y <= 0 && m > x && m > y)
    cout << "-1" << endl;
  else {
    if (x < 0 && x < m && y < m) cnt = abs(x) / y, x += cnt * y;
    if (y < 0 && x < m && y < m) cnt = abs(y) / x, y += cnt * x;
    cout << solve(x, y) + cnt << endl;
  }
  return 0;
}
