```go
package main

import (
    "fmt"
)

func main() {
    var s, t string
    fmt.Scan(&s, &t)
    
    result := []byte(s)
    n := len(result)
    
    i := n - 1
    for i >= 0 && result[i] == 'z' {
        result[i] = 'a'
        i--
    }
    if i < 0 {
        fmt.Println("No such string")
        return
    }
    result[i]++
    
    if string(result) < t {
        fmt.Println(string(result))
    } else {
        fmt.Println("No such string")
    }
}
```