#include <bits/stdc++.h>
using namespace std;
int main() {
  long long x, y, m;
  while (cin >> x >> y >> m) {
    if (x >= m || y >= m)
      cout << 0 << endl;
    else if (x <= 0 && y <= 0)
      cout << -1 << endl;
    else {
      long long cnt = 0;
      long long xx = min(x, y);
      long long yy = max(x, y);
      if (xx < 0) {
        cnt = (-xx) / yy + 1;
        xx = cnt * yy + xx;
      }
      while (yy < m) {
        x = xx + yy;
        y = yy;
        xx = min(x, y);
        yy = max(x, y);
        cnt++;
      }
      cout << cnt << endl;
    }
  }
  return 0;
}
