#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#include<cmath>
#include<vector>
#include<queue>
using namespace std;

#define X first
#define Y second

typedef pair<int,int> pii;
typedef long long LL;
const char nl = '\n';
const int N = 1e6+10;
const int M = 1e6+10;
LL x,y,k,n;
LL t;

void solve(){
	LL ans = 0,m = 0,num = 0;
	cin >> x >> y >> k >> n >> t;
	int ox = x;
	while(n){
		ans += n*x;
		m += n;
		x = ox + y * (m/k);
		n --;
		num ++;
		if(ans >= t)break;
	}
	if(ans >= t)cout << num;
	else cout << -1;

}

int main(){
	ios::sync_with_stdio(false);
	cin.tie(0),cout.tie(0);

	solve();
}