package main

import (
	"fmt"
	"strings"
)

func main() {
	var s string
	fmt.Scan(&s)
	c4 := strings.Count(s, "4")
	c7 := strings.Count(s, "7")
	if c4 == 0 && c7 == 0 {
		fmt.Println("-1")
	} else if c4 >= c7 {
		fmt.Println("4")
	} else {
		fmt.Println("7")
	}
}