Buggy python submission (verdict: Runtime Error):
#    !/usr/bin/env python3
#    encoding: UTF-8
#    Modified: <22/Jun/2019 05:50:52 PM>


#    ✪ H4WK3yE乡
#    Mohd. Farhan Tahir
#    Indian Institute Of Information Technology (IIIT), Gwalior


import sys
import os
from io import IOBase, BytesIO


def main():
    a, b, c, d = get_ints()
    l = 0
    if a % c == 0:
        l = a
    else:
        l = (a // c) * c + c
    r = (b // c) * c
    n1 = (r - l) // c + 1

    if a % d == 0:
        l = a
    else:
        l = (a // d) * d + d
    r = (b // d) * d
    n2 = (r - l) // d + 1

    from math import gcd
    g = (c * d) // gcd(c, d)
    if a % g == 0:
        l = a
    else:
        l = (a // g) * g + g
    r = (b // g) * g
    n3 = (r - l) // g + 1

    print(b - a + 1 - (n1 + n2 - n3))


BUFSIZE = 8192


class FastIO(BytesIO):
    newlines = 0

    def __init__(self, file):
        self._file = file
        self._fd = file.fileno()
        self.writable = "x" in file.mode or "w" in file.mode
        self.write = super(FastIO, self).write if self.writable else None

    def _fill(self):
        s = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
        self.seek((self.tell(), self.seek(0, 2), super(FastIO, self).write(s))[0])
        return s

    def read(self):
        while self._fill():
            pass
        return super(FastIO, self).read()

    def readline(self):
        while self.newlines == 0:
            s = self._fill()
            self.newlines = s.count(b"\n") + (not s)
        self.newlines -= 1
        return super(FastIO, self).readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.getvalue())
            self.truncate(0), self.seek(0)


class IOWrapper(IOBase):
    def __init__(self, file):
        py2 = round(0.5)
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        if py2 == 1:
            self.write = self.buffer.write
            self.read = self.buffer.read
            self.readline = self.buffer.readline
        else:
            self.write = lambda s: self.buffer.write(s.encode("ascii"))
            self.read = lambda: self.buffer.read().decode("ascii")
            self.readline = lambda: self.buffer.readline().decode("ascii")


sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)


def get_array(): return list(map(int, sys.stdin.readline().split()))


def get_ints(): return map(int, sys.stdin.readline().split())


def input(): return sys.stdin.readline().strip()


if __name__ == "__main__":
    main()


Fixed submission:
#    !/usr/bin/env python3
#    encoding: UTF-8
#    Modified: <22/Jun/2019 05:52:43 PM>


#    ✪ H4WK3yE乡
#    Mohd. Farhan Tahir
#    Indian Institute Of Information Technology (IIIT), Gwalior


import sys
import os
from io import IOBase, BytesIO


def main():
    a, b, c, d = get_ints()
    l = 0
    if a % c == 0:
        l = a
    else:
        l = (a // c) * c + c
    r = (b // c) * c
    n1 = (r - l) // c + 1

    if a % d == 0:
        l = a
    else:
        l = (a // d) * d + d
    r = (b // d) * d
    n2 = (r - l) // d + 1

    from fractions import gcd
    g = (c * d) // gcd(c, d)
    if a % g == 0:
        l = a
    else:
        l = (a // g) * g + g
    r = (b // g) * g
    n3 = (r - l) // g + 1

    print(b - a + 1 - (n1 + n2 - n3))


BUFSIZE = 8192


class FastIO(BytesIO):
    newlines = 0

    def __init__(self, file):
        self._file = file
        self._fd = file.fileno()
        self.writable = "x" in file.mode or "w" in file.mode
        self.write = super(FastIO, self).write if self.writable else None

    def _fill(self):
        s = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
        self.seek((self.tell(), self.seek(0, 2), super(FastIO, self).write(s))[0])
        return s

    def read(self):
        while self._fill():
            pass
        return super(FastIO, self).read()

    def readline(self):
        while self.newlines == 0:
            s = self._fill()
            self.newlines = s.count(b"\n") + (not s)
        self.newlines -= 1
        return super(FastIO, self).readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.getvalue())
            self.truncate(0), self.seek(0)


class IOWrapper(IOBase):
    def __init__(self, file):
        py2 = round(0.5)
        self.buffer = FastIO(file)
        self.flush = self.buffer.flush
        self.writable = self.buffer.writable
        if py2 == 1:
            self.write = self.buffer.write
            self.read = self.buffer.read
            self.readline = self.buffer.readline
        else:
            self.write = lambda s: self.buffer.write(s.encode("ascii"))
            self.read = lambda: self.buffer.read().decode("ascii")
            self.readline = lambda: self.buffer.readline().decode("ascii")


sys.stdin, sys.stdout = IOWrapper(sys.stdin), IOWrapper(sys.stdout)


def get_array(): return list(map(int, sys.stdin.readline().split()))


def get_ints(): return map(int, sys.stdin.readline().split())


def input(): return sys.stdin.readline().strip()


if __name__ == "__main__":
    main()