#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,cmp,cmpp,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;};

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

int main(void){
  
       cin.tie(0);
    ios::sync_with_stdio(false);
      
      cin>>a>>b>>c;
      rep(i,200000){
        cmp=a*60*i;
        cmpp=c*3600+b*i;
        if(cmp>=cmpp){
          Cout(i);
          return 0;
        }
      }
      Cout(-1);
    
       
return 0;
}