#include<bits/stdc++.h> using namespace std; #define ll long long #define FOR(i,n,m) for(int i=(n);i<(m);i++) #define REP(i,n) FOR(i,0,n) #define REPR(i,n) for(int i=(n);i>=0;i--) #define all(vec) vec.begin(),vec.end() using vi=vector<int>; using vvi=vector<vi>; using vl=vector<ll>; using vvl=vector<vl>; using P=pair<int,int>; using PP=pair<int,P>; using Pl=pair<ll,ll>; using PPl=pair<ll,Pl>; using vs=vector<string>; #define fi first #define se second #define pb push_back template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;} template<class T>bool chmin(T &a,const T &b){if(a>b){a=b;return true;}return false;} const ll MOD=1000000007LL; const int INF=1<<30; const ll LINF=1LL<<60; int main(){ ll a,b,c; cin>>a>>b>>c; a*=60; c*=3600; if(a<=b){ cout<<-1<<endl; return 0; } cout<<(c+a-b-1)/(a-b)<<endl; return 0; }