```go
package main

import (
	"fmt"
)

func main() {
	var a, ta int
	var b, tb int
	var s string
	fmt.Scan(&a, &ta)
	fmt.Scan(&b, &tb)
	fmt.Scan(&s)

	hh := (s[0] - '0') * 10 + (s[1] - '0')
	mm := (s[3] - '0') * 10 + (s[4] - '0')
	S := int(hh)*60 + int(mm)

	left := 300
	if S-tb+1 > left {
		left = S - tb + 1
	}
	right := 1439
	if S+ta-1 < right {
		right = S + ta - 1
	}

	if left > right {
		fmt.Println(0)
		return
	}

	kMin := (left - 300 + b - 1) / b
	kMax := (right - 300) / b

	if kMax < kMin {
		fmt.Println(0)
	} else {
		fmt.Println(kMax - kMin + 1)
	}
}
```