#include <bits/stdc++.h>
using namespace std;
int main() {
  long long a, b, m, k, l;
  cin >> a >> b >> m;
  if (a >= m || b >= m)
    cout << 0;
  else {
    long long sum = a + b;
    if (sum > a || sum > b) {
      k = 0;
      if (a < 0) {
        l = -a / b;
        a = a + l * b;
        k += l;
      } else if (b < 0) {
        l = -b / a;
        b = b + l * a;
        k += l;
      }
      while (true) {
        k++;
        sum = a + b;
        if (a < b)
          a = sum;
        else
          b = sum;
        if (sum >= m) {
          cout << k;
          break;
        }
      }
    } else {
      cout << -1;
    }
  }
  cout << endl;
  return 0;
}
