#include <bits/stdc++.h>
using namespace std;
//#include <atcoder/all>
//using namespace atcoder;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vvvvll = vector<vvvll>;
using vvvvvll = vector<vvvvll>;
using vvvvvvll = vector<vvvvvll>;
using vb = vector<bool>;
using vvb = vector<vb>;
using vvvb = vector<vvb>;
using vvvvb = vector<vvvb>;
using vd = vector<double>;
using vvd = vector<vd>;
using vvvd = vector<vvd>;
using vvvvd = vector<vvvd>;
using vvvvvd = vector<vvvvd>;
#define all(A) A.begin(),A.end()
#define ALL(A) A.begin(),A.end()
#define rep(i, n) for (ll i = 0; i < (ll) (n); i++)
using pqr = priority_queue<pair<ll, ll>, vector<pair<ll, ll>>, greater<pair<ll, ll>>>;
#define tN(t) (t==1?XN:YN)
#define tS(t) (t==1?XS:YS)
#define tA(t) (t==1?XA:YA)

template<class T>
bool chmax(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p < q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}

template<class T>
bool chmin(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p > q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}

ll gcd(ll(a), ll(b)) {
    if (b == 0)return a;
    ll c = a;
    while (a % b != 0) {
        c = a % b;
        a = b;
        b = c;
    }
    return b;
}
ll sqrtz(ll N) {
    ll L = 0;
    ll R = sqrt(N) + 10000;
    while (abs(R - L) > 1) {
        ll mid = (R + L) / 2;
        if (mid * mid <= N)L = mid;
        else R = mid;
    }
    return L;

}

void d(vb &AB, string &A, string &B) {
    rep(i, A.size()) {
        bool OK = 1;
        for (ll j = 0; j < B.size(); j++) {
            if (i + j >= A.size())break;
            if (A[i + j] == '?' || B[j] == '?')continue;
            if (A[i + j] != B[j])OK = 0;
        }
        AB[i] = OK;
    }
}
ll res = 1e18;
void f(string &A, string &B, string &C, vb &AB, vb &BC, vb &AC) {
    
    rep(i, A.size() + 1) {
        if (!AB[i])continue;
        rep(j, A.size() + B.size() + 2) {
            if (!BC[j])continue;
            if (!AC[i + j])continue;
            ll L=ll(max({ll(i+j+C.size()),ll(i+B.size()),ll(A.size())}));
            chmin(res, L);
        }
    }
    return;
}

const ll mod = 998244353;
int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    ll A,B,C;
    cin>>A>>B>>C;
    if(60*A<=B){
        cout<<-1<<endl;
        return 0;
    }
    ll D=60*A-B;
    cout<<(3600*C+D-1)/D<<endl;
}