#include <bits/stdc++.h>
using namespace std;
void fastInOut() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL), cout.tie(NULL);
}
int dx[] = {0, 0, 1, -1, -1, -1, 1, 1};
int dy[] = {1, -1, 0, 0, -1, 1, -1, 1};
long long n, m, k;
long long cnt = 0;
void solve(long long x, long long y) {
  if (x >= k || y >= k) return;
  cnt++;
  if (x >= y)
    solve(x, y + x);
  else
    solve(x + y, y);
}
int main() {
  cin >> n >> m >> k;
  if (n < k && m < k && n <= 0 && m <= 0) {
    cout << -1;
    return 0;
  }
  long long a = min(n, m);
  long long b = max(n, m);
  if (b < k) {
    if (a < 0) {
      cnt += ((-a) / b);
      if ((-a) % b != 0) cnt++;
      a = b - ((-a) % b);
      if (a == b) a = 0;
    }
    n = a;
    m = b;
    solve(n, m);
  }
  cout << cnt;
}
