public static void main ( String args [ ] ) throws IOException {
  Reader sc = new Reader ( ) ;
  int c = sc . nextInt ( ) ;
  int v0 = sc . nextInt ( ) ;
  int v1 = sc . nextInt ( ) ;
  int a = sc . nextInt ( ) ;
  int l = sc . nextInt ( ) ;
  int d = 1 ;
  int t = c ;
  c -= v0 ;
  if ( c > 0 ) {
    c = Math . min ( t , c + l ) ;
    int s = Math . min ( v0 + d * a , v1 ) ;
    while ( c > 0 ) {
      c -= s ;
      d ++ ;
      if ( c <= 0 ) {
        break ;
      }
      s = Math . min ( v0 + d * a , v1 ) ;
      c = Math . min ( t , c + l ) ;
    }
  }
  out . println ( d ) ;
  out . close ( ) ;
}

public static void main ( String args [ ] ) throws IOException { Reader sc = new Reader ( ) ; int c = sc . nextInt ( ) ; int v0 = sc . nextInt ( ) ; int v1 = sc . nextInt ( ) ; int a = sc . nextInt ( ) ; int l = sc . nextInt ( ) ; int d = 1 ; int t = c ; c -= v0 ; if ( c > 0 ) { c = Math . min ( t , c + l ) ; int s = Math . min ( v0 + d * a , v1 ) ; while ( c > 0 ) { c -= s ; d ++ ; if ( c <= 0 ) { break ; } s = Math . min ( v0 + d * a , v1 ) ; c = Math . min ( t , c + l ) ; } } out . println ( d ) ; out . close ( ) ; }
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