#include <bits/stdc++.h>

#define int long long

using namespace std;
#define rep(i,s,n) for(int i = s;i<n;i++)
#define repe(i,s,n) for(int i = s;i<=n;i++)
#define pb push_back
#define fi first
#define se second
typedef long long ll;
typedef pair<int,int>pint;
typedef vector<int>vint;
typedef vector<pint>vpint;
static const ll maxLL = (ll)1 << 62;
const ll MOD=1000000007,INF=1e18;
int dy[]={-1,0,1,0};
int dx[]={0,1,0,-1};

int a,b,c;

signed main(){
    cin>>a>>b>>c;
    if(60*a<=b)cout<<-1;
    else cout<<(ll)ceil((double)3600*c/(60*a-b));
    
    
    
    
    
    
    
    
    
    return 0;
}