#include <bits/stdc++.h>
using namespace std;
long long x, y, m, aux, rs = 0;
void neg() {
  rs = (-y) / x;
  rs++;
  y = y + rs * x;
}
void rec() {
  aux = x + y;
  rs++;
  if (aux < m) {
    y = x;
    x = aux;
    rec();
  }
}
int main() {
  cin >> x >> y >> m;
  aux = max(x, y);
  y = min(x, y);
  x = aux;
  if (x >= m) {
    cout << "0" << endl;
    return 0;
  }
  if (x < 1) {
    cout << "-1" << endl;
    return 0;
  }
  if (y < 1 && x > 0) {
    neg();
  }
  rec();
  cout << rs << endl;
  return 0;
}
