#include <iostream>
using namespace std;
int c,v,vl,l,a;
int main(){
cin>>c>>v>>vl>>a>>l;
int s=1;
for(int i=v;i<c;){
v+=a;
if(v>vl) v=vl;
i-=l;
i+=v;
s++;}
cout<<s<<endl;
}