#include <bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<(b);++i)
#define erep(i,a,b) for(int i=a;i<=(int)(b);++i)
#define per(i,a,b) for(int i=(a);i>(b);--i)
#define eper(i,a,b) for(int i=(a);i>=b;--i)
#define pb push_back
#define mp make_pair
#define INF 100100100100
#define MOD 1000000007
#define ALL(x) begin(x),end(x)
using namespace std;
typedef long long ll;
typedef pair<int,int> Pii;
typedef vector<int> vii;
typedef vector<ll> vll;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a = b; return 1; } return 0; }
int dy[]={0, 0, 1, -1};
int dx[]={1, -1, 0, 0};
int gcd(int a,int b){return b?gcd(b,a%b):a;}
int lcm(int a,int b){return a/gcd(a, b)*b;}

double a, b, c;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
    cin >> a >> b >> c;
    if (60.0 * a - b < 0) printf("-1\n");
    else printf("%d\n", (int)ceil((60.0 * 60.0 * c) / (60.0 * a - b)));
    return 0;
}