from collections import deque

n, m, a, b = map(int, input().split())
n = b - a + 2
edges = [[] for _ in range(n)]
for _ in range(m):
    l, r = map(int, input().split())
    if l <= a and b <= r:
        print(1)
        exit()
    elif r < a or b < l:
        pass
    else:
        l = max(l, a) - a
        r = min(r, b) - a + 1
        edges[l].append(r)
        edges[r].append(l)
        
        
queue = deque()
queue.append(0)
dist = [-1] * n
dist[0] = 0
while queue:
    pos = queue.popleft()
    for npos in edges[pos]:
        if dist[npos] != -1:
            continue
        dist[npos] = dist[pos] + 1
        queue.append(npos)
print(dist[-1])