#include <bits/stdc++.h>
using namespace std;

using i32 = int32_t;using i64 = int64_t;using u32 = uint32_t;using u64 = uint64_t;using f32 = float;using f64 = double;using f80 = long double;
using i128 = __int128_t;using u128 = __uint128_t;
using vi32 = vector<i32>;using vi64 = vector<i64>;using vu32 = vector<u32>;using vu64 = vector<u64>;
using vvi32 = vector<vector<int32_t>>;using vvi64 = vector<vector<int64_t>>;using vvu32 = vector<vector<uint32_t>>;using vvu64 = vector<vector<uint64_t>>;
using pi32 = pair<i32,i32>;using pi64 = pair<i64,i64>;using pu32 = pair<u32,u32>;using pu64 = pair<u64,u64>;
using vpi32 = vector<pi32>;using vpi64 = vector<pi64>;using vpu32 = vector<pu32>;using vpu64 = vector<pu64>;

int main() {
  i64 A, B, C;
  cin >> A >> B >> C;
  A *= 60;
  C *= 3600;
  if (B < A) cout << ((C + A - B - 1) / (A - B)) << '\n';
  else cout << "-1\n";
  return 0;
}