#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)
#define revrep(i,a,b) for(int i=a;i>b;i--)
#define int long long

const int inf = 100100100100000;
const int mod = 1000000007;
int dx[4]={1,0,-1,0};
int dy[4]={0,1,0,-1};

int input(){int tmp;cin >> tmp;return tmp;}
string raw_input(){string tmp;cin >> tmp;return tmp;}
string readline(){string s;getline(cin, s);return s;}
void print(int n){cout << n << endl;}
void print(vector<int> arr){cout << arr[0];rep(i,1,arr.size()) cout << " " << arr[i];cout << endl;}
void print(string s){cout << s << endl;}
void print(pair<int, int> p){cout << p.first << " " << p.second;}
void print(vector<vector<int>> arr){rep(i,0,arr.size()){print(arr[i]);}}
void print(double n){cout << n << endl;}
int ceil(int a, int b){if(a%b == 0) return a/b;else return a/b + 1;}

//main-----------------------------------------------------------------------------


signed main(){
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    int a,b,c; cin >> a >> b >> c;
    a *= 60;
    c *= 3600;
    

    int x = c/(a-b);
    print( x + 1 > 0 ? x + 1 : -1);





}