Go: package main

import (
	"fmt"
	"bufio"
	"os"
	"strconv"
	"strings"
)

var sc = bufio.NewScanner(os.Stdin)

func nextLine() string {
	sc.Scan()
	return sc.Text()
}

func nextLineInt() []int {
	arr := strings.Split(nextLine(), " ")
	var ret []int
	for _, v := range arr {
		x, _ := strconv.Atoi(v)
		ret = append(ret, x)
	}
	return ret
}

func main() {
	cin := nextLineInt()
	F := cin[0]
	fmt.Println((F-30)/2)
}

C++: #include <cassert>

#include <cctype>
#include <cerrno>
#include <cfloat>
#include <ciso646>
#include <climits>
#include <clocale>
#include <cmath>
#include <csetjmp>
#include <csignal>
#include <cstdarg>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <ccomplex>
#include <cfenv>
#include <cinttypes>
#include <cstdbool>
#include <cstdint>
#include <ctgmath>
#include <cwchar>
#include <cwctype>

// C++
#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <array>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <forward_list>
#include <future>
#include <initializer_list>
#include <mutex>
#include <random>
#include <ratio>
#include <regex>
#include <scoped_allocator>
#include <system_error>
#include <thread>
#include <tuple>
#include <typeindex>
#include <type_traits>
#include <unordered_map>
#include <unordered_set>
//#include <boost/foreach.hpp>
//#include <boost/range/algorithm.hpp>
#define rep(i,j,k) for(int i=(int)j;i<(int)k;i++)
#define ll long long
#define Sort(v) sort(all(v))
#define INF 1000000000
#define END return 0
#define pb push_back
#define se second
#define fi first
#define pb push_back
#define all(v) (v).begin() , (v).end()
#define MP make_pair
#define MOD 1000000007LL
using namespace std;
int day[12]={31,28,31,30,31,30,31,31,30,31,30,31};

int main (){ 
    int f;
    cin>>f;
    cout<<f/2-15<<endl;
    
}


/*
abcde

/root
a.txt
bunp.html
/desktop
r.txt
fsharp.cs
dp.cpp
/users
bison.txt

root r.txt
users r.txt
users a.txt

desktop/r.txt
../r.txt
../../a.txt
*/

C++: #include <iostream>
using namespace std;
int main(void){
    int n;
    cin>>n;
    cout<<(n-30)/2;
    cout<<endl;
    
}

C++: #include <iostream>

using namespace std;

int main(){
  int f;cin>>f;
  cout<<(f-30)/2<<endl;
}

C++: #include <iostream>
using namespace std;
int main(){
    int C;
    cin >> C;
    cout << ( C - 30 ) / 2 << endl;
    return 0;
    }

C++: #include<bits/stdc++.h>
using namespace std;

int main(void){
    int a;
    cin >> a;
    cout << (a-30)/2 <<endl;
    return 0;
}

C++: #include <iostream>

using namespace std;

int main(){
    int f;
    cin >> f;
    cout << (f - 30) / 2 << "\n";
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
using namespace std;
int main() {
	int f;
	cin >> f;
	cout << (f - 30) / 2 << endl;
}