#include <bits/stdc++.h>
using namespace std;
int main() {
  long long X, Y, x, y, aux, m, z = 0LL, t;
  while (cin >> X >> Y >> m) {
    x = max(X, Y), y = min(X, Y);
    if (x >= m) {
      puts("0");
      continue;
    }
    if (x < z) {
      puts("-1");
      continue;
    }
    if (x == z) {
      puts("-1");
      continue;
    }
    if (x > z && y < z) {
      t = abs(y / x);
      y = y + (1LL * x * t);
      while (max(x, y) < m) {
        t++;
        aux = x + y;
        x = max(x, y);
        y = aux;
      }
    } else {
      t = 0;
      while (max(x, y) < m) {
        t++;
        aux = x + y;
        x = max(x, y);
        y = aux;
      }
    }
    cout << t << endl;
  }
  return 0;
}
