#include <bits/stdc++.h>
using namespace std;
int Get() {
  char c;
  while (c = getchar(), (c < '0' || c > '9') && (c != '-'))
    ;
  bool Flag = (c == '-');
  if (Flag) c = getchar();
  int X = 0;
  while (c >= '0' && c <= '9') {
    X = X * 10 + c - 48;
    c = getchar();
  }
  return Flag ? -X : X;
}
void Output(int X) {
  if (X < 0) {
    putchar('-');
    X = -X;
  }
  int Len = 0, Data[10];
  while (X) {
    Data[Len++] = X % 10;
    X /= 10;
  }
  if (!Len) Data[Len++] = 0;
  while (Len--) putchar(Data[Len] + 48);
  putchar('\n');
}
int main() {
  long long X, Y, M;
  cin >> X >> Y >> M;
  if (X >= M || Y >= M) {
    cout << 0 << endl;
    return 0;
  }
  if (X < Y) swap(X, Y);
  if (X <= 0) {
    cout << -1 << endl;
    return 0;
  }
  long long Ans = 0;
  if (Y < 0) {
    Ans = -Y / X;
    Y += Ans * X;
  }
  while (X < M && Y < M) {
    if (X < Y)
      X += Y;
    else
      Y += X;
    Ans++;
  }
  cout << Ans << endl;
  return 0;
}
