In Japan, temperature is usually expressed using the Celsius (℃) scale. In America, they used the Fahrenheit (℉) scale instead. $20$ degrees Celsius is roughly equal to $68$ degrees Fahrenheit. A phrase such as "Today’s temperature is $68$ degrees" is commonly encountered while you are in America.

A value in Fahrenheit can be converted to Celsius by first subtracting $32$ and then multiplying by $\frac{5}{9}$. A simplified method may be used to produce a rough estimate: first subtract $30$ and then divide by $2$. Using the latter method, $68$ Fahrenheit is converted to $19$ Centigrade, i.e., $\frac{(68-30)}{2}$.

Make a program to convert Fahrenheit to Celsius using the simplified method: $C = \frac{F - 30}{2}$.



Input

The input is given in the following format.


$F$


The input line provides a temperature in Fahrenheit $F$ ($30 \leq F \leq 100$), which is an integer divisible by $2$.

Output

Output the converted Celsius temperature in a line.

Examples

Input

68


Output

19


Input

50


Output

10
#include <bits/stdc++.h>

#define rep(i, n) for (int(i) = 0; (i) < (int)(n); (i)++)
#define repr(i, n) for (int(i) = (n); (i) >= 0; (i)--)
#define repf(i, m, n) for (int(i) = (m); (i) < (n); (i)++)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)(x).size())
#define inf 1e9

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;
}

using namespace std;
using ll = long long;

int kakuinowa(int num, int a, int n)
{
    string tmp = to_string(num);
    vector<int> keta(10, 0);

    for (int i = 0; i < tmp.size(); i++)
    {
        keta[i] = tmp[i] - '0';
    }

    ll sum = accumulate(all(keta), 0);

    return pow(sum + a, n);
}

int main(int argc, char const *argv[])
{
    int f;
    cin >> f;
    cout << (f - 30) / 2 << endl;

    return 0;
}

