#include<iostream>
#include<vector>
#include<queue>
#include<stack>
#include<string>
#include<cmath>
#include<algorithm>
#include<bitset>
#include<map>
#include<functional>
#include<set>
#include<numeric>

#pragma region
using namespace std;
#define FOR(i,r,n) for(ll i = (ll)(r); i < (ll)(n); i++)
#define rep(i,n) FOR(i,0LL,n)
#define RFOR(i,r,n) for(ll i=(ll)(n-1);i>=r;i--)
#define rrep(i,n) RFOR(i,0LL,n)
#define all(x) x.begin(),x.end()
#define rall(x) x.rbegin(),x.rend()
#define COUNT(a,y,x) upper_bound(all(a), y) - lower_bound(all(a), x)
#define UNIQUE(a) sort(all(a)); a.erase(unique(all(a)), a.end())
#define SUM(a) accumulate(all(a),0LL)
#define scan(x) scanf("%lld",&x)
#define print(x) printf("%lld\n",x)
#define print2(x) printf("%lld ",x)
#define pb push_back
typedef long long int ll;
typedef vector<ll> vll;
typedef vector<vll> vvll;
typedef pair<ll, ll> pll;
typedef vector<pll> vpll;
typedef vector<string> vs;
typedef map<ll, ll> MAP;
const ll inf = 2222222222222222222LL;
const ll mod = 1000000007LL;

ll n = 0, m = 0, ans = 0, cnt = 0, tmp = 0, ma = -inf, mi = inf;
string s;
bool ok = true, flag = false;
ll dx[5] = { 1,-1,0,0,0 }, dy[5] = { 0,0,1,-1,0 };
ll ddx[9] = { 1,-1,0,0,1,1,-1,-1,0 }, ddy[9] = { 0,0,1,-1,1,-1,1,-1,0 };
#pragma endregion
#define MAX 222222



int main(void) {
  ll a, b, c;
  cin >> a >> b >> c;
  a *= 60;
  c *= 3600;
  if (a <= b) cout << "-1" << endl;
  else {
	ll up = mod*100, mid, low = 0;
	while (up - low > 1) {
	  mid = (up + low) / 2;
	  if (a*mid >= b * mid + c) up = mid;
	  else low = mid;
	}
	cout << up << endl;
  }


  return 0;
}