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