#include <bits/stdc++.h>
using namespace std;
long long x, y, m, i = 0, temp;
int main() {
  scanf("%I64d %I64d %I64d", &x, &y, &m);
  if (x > y) swap(x, y);
  if (x <= 0 && y <= 0) {
    if (y >= m)
      printf("0\n");
    else
      printf("-1\n");
  } else {
    i = 0;
    while (y < m) {
      if (x >= 0) {
        x += y;
        swap(x, y);
        i++;
      } else {
        temp = (-x + y - 1) / y;
        x += temp * y;
        i += temp;
      }
    }
    printf("%I64d\n", i);
  }
  return 0;
}
