#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int x, y, m;
  cin >> x >> y >> m;
  if (x >= m || y >= m) {
    printf("0\n");
    return 0;
  }
  if (x <= 0 && y <= 0) {
    printf("-1\n");
    return 0;
  }
  long long int ct = 0;
  if (x <= 0) swap(x, y);
  if (y < 0) {
    if (m < 0) {
      cout << (m - y + x - 1) / x << endl;
      return 0;
    } else {
      ct = (-y + x - 1) / x;
      y += x * ct;
    }
  }
  while (x < m && y < m) {
    long long int sm = x + y;
    if (x < y)
      x = sm;
    else
      y = sm;
    ct++;
  }
  cout << ct << endl;
  return 0;
}
