/*
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*/
#include<bits/stdc++.h>
using namespace std;
#define ALL(obj) (obj).begin(), (obj).end()
#define REP(i, n) for(int i = 0;i < (n);++i)

#define endl '\n'
#define pb emplace_back
#define mkp make_pair
#define p_q priority_queue
#define MOD 1000000007LL
#define INF 2000000000
#define PI 3.1415926

const int dx[]={1,0,-1,0}, dy[]={0,-1,0,1};
const int dx_8[]={1,1,0,-1,-1,-1,0,1}, dy_8[]={0,-1,-1,-1,0,1,1,1};

typedef long long ll;
// #define int long long

template <typename T> inline T GCD(T a,T b){T c;while(b!=0){c=a%b;a=b;b=c;}return a;}
template <typename T> inline T LCM(T a,T b){T c=GCD(a,b);a/=c;return a*b;}
template <typename T> inline T nCr(T a,T b){T i,r=1;for(i=1;i<=b;i++){r*=(a+1-i);r/=i;}return r;}
template <typename T> inline T nHr(T a,T b){return nCr(a+b-1,b);}

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

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

  return 0;
}


Measured memory_bytes: 3200