#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
  #define _GLIBCXX_DEBUG
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  ll a, b, c;
  cin >> a >> b >> c;
  // 60ax >= 3600c + bx
  // 60ax - bx >= 3600c
  // x(60a - b) >= 3600c
  // x >= 3600c / (60a - b)
  ll bunbo = 60 * a - b;
  if (bunbo <= 0) {
    cout << -1 << endl;
    return 0;
  }
  cout << (3600 * c + bunbo - 1) / bunbo << endl;
  return 0;
}