#include <bits/stdc++.h>
using namespace std;
long long x, y, m;
int main() {
  cin >> x >> y >> m;
  if (x >= m || y >= m)
    cout << 0 << '\n';
  else {
    if (x <= 0 && y <= 0)
      cout << -1 << '\n';
    else {
      long long cc = 0;
      if (x > y) swap(x, y);
      if (x < 0) {
        cc += x >= 0 ? x : -x / y;
        x += y * (long long)cc;
      }
      while (x < m && y < m) {
        if (x < y)
          x = x + y;
        else
          y = x + y;
        cc++;
      }
      cout << cc << '\n';
    }
  }
}
