#include <bits/stdc++.h>

#define rep(i,n) for(long long int (i)=0;(i)<(int)(n);(i)++)
#define rrep(i,a,b) for(long long int i=(a);i<(b);i++)
#define rrrep(i,a,b) for(long long int i=(a);i>=(b);i--)
#define all(v) (v).begin(), (v).end()
#define pb(q) push_back(q)
#define Size(n) (n).size()
#define Cout(x) cout<<(x)<<endl

typedef long long ll;

using namespace std;

const int INF = 1e9,MOD = 1e9 + 7,ohara = 1e6;
const ll LINF = 1e18;
long long int n,cnt=0,ans=0,a,b,c,d,m,h,w,x,y,sum=0,pos;
int dy[]={1,0,-1,0};
int dx[]={0,1,0,-1};
string alph("abcdefghijklmnopqrstuvwxyz"),s;
bool fl;
struct edge{int to,cost;};

//-------------------------↓↓↓↓↓↓------------------------

bool check(ll z){
  double cmp=(double)((a/60.0)*z);
  double cmpp=(double)((b/3600.0)*z+c);
  //cmp=((double)a/60.0)*(double)z)
  //cout<<cmp<<" "<<cmpp<<"\n"; 
  if(cmp>=cmpp)return true;
  else return false;
}

int main(void){
  
       cin.tie(0);
    ios::sync_with_stdio(false);
      
      cin>>a>>b>>c;
      ll le=0,ri=LINF,mid;
      while(ri-le>1){
        mid=(le+ri)/2;
        if(check(mid)){
          ri=mid;
        }
        else{
          le=mid;
        }
      }
     if(ri==LINF)Cout(-1);
     else Cout(ri);
    
       
return 0;
}