mod = 1000000007
eps = 10**-9


def main():
    import sys
    from collections import deque
    input = sys.stdin.buffer.readline

    N, M, A, B = map(int, input().split())
    N += 1
    adj = [set() for _ in range(N+1)]
    for _ in range(M):
        l, r = map(int, input().split())
        r += 1
        l = max(l, A)
        r = min(r, B+1)
        adj[l].add(r)
        adj[r].add(l)

    que = deque()
    que.append(A)
    seen = [-1] * (N+1)
    seen[A] = 0
    par = [0] * (N+1)
    child = [[] for _ in range(N+1)]
    seq = []
    while que:
        v = que.popleft()
        seq.append(v)
        for u in adj[v]:
            if seen[u] == -1:
                seen[u] = seen[v] + 1
                par[u] = v
                child[v].append(u)
                que.append(u)
    seq.reverse()

    print(seen[B+1])


if __name__ == '__main__':
    main()