#include <bits/stdc++.h>
using namespace std;
long n, v, d = 0, c, m1;
struct o {
  long x;
  long y;
};
int b[1000005], inf = 100000000;
vector<int> a[100001];
long l[105][105] = {0}, s = 0;
bool used[100001];
bool func(o a, o b) { return a.x < b.x; }
int m, mine[100001], ind[105], raod = 0;
void dfs(int v) {
  used[v] = 1;
  mine[v] = raod;
  for (int k = 0; k < a[v].size(); ++k)
    if (!used[a[v][k]]) dfs(a[v][k]);
}
long long x, y, z;
int main() {
  cin >> x >> y >> z;
  if (x >= z || y >= z) {
    cout << 0;
    return 0;
  }
  if (x <= 0 && y <= 0) {
    cout << -1;
    return 0;
  }
  long long t = max(x, y);
  long long u = min(x, y);
  long long raod = 0;
  if (u < 0) {
    raod += abs(u / t);
    if (u % t != 0) ++raod;
    u = u + raod * t;
  }
  t = max(t, u);
  u = min(t, u);
  while (t < z) {
    u = t + u;
    ++raod;
    if (u > t) swap(u, t);
  }
  cout << raod;
  return 0;
}
