#include<bits/stdc++.h>
using namespace std;
#define ALL(x) x.begin(),x.end()
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define INF 1000000000
#define mod 1000000007
using ll=long long;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0};
int dy[]={0,1,0,-1};
// ll gcd(ll a,ll b){return b?gcd(b,a%b):a;}
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(b<a){a=b;return true;}return false;}

using int128=__int128;
 
ostream &operator<<(ostream &os,const __int128 n){
    if(n==0){
        os<<0;
        return os;
    }
    __int128 num=n;
    bool neg=false;
    if(num<0)neg=true,num=-num;
    string res="";
    while(num>0){
        res.push_back('0'+num%10);
        num/=10;
    }
    if(neg) res.push_back('-');
    reverse(begin(res),end(res));
    os<<res;
    return os;
}
 
istream &operator>>(istream &is,__int128 &n){
    string s;
    is>>s;
    int idx=0;
    bool neg=false;
    if(s[0]=='-')neg=true,idx++;
    n=0;
    for(;idx<(int)s.size();idx++){
        n=10*n+s[idx]-'0';
    }
    if(neg) n=-n;
    return is;
}
 
__int128 abs(__int128 x){
    return x<0?-x:x;
}
 
__int128 gcd(__int128 a,__int128 b){
    if(a==0) return b;
    if(b==0) return a;
    return b>0?gcd(b,a%b):a;
}
 



signed main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int128 a,b,c;cin>>a>>b>>c;
    a*=60;
    c*=3600;
    if(a<=b){
        cout<<-1<<endl;
        return 0;
    }

    int128 lw=0,hi=LINF,res;
    while(lw<=hi){
        int128 m=(lw+hi)/2;
        int128 hand=a*m;
        int128 machine=c+b*m;
        if(machine<=hand){
            res=m;
            hi=m-1;
        }else{
            lw=m+1;
        }
    }
    cout<<res<<endl;
    return 0;
}