from sys import stdin
def kg():
    n,m = map(int,input().split())
    qw = [[[0,0] for i in range(n+1)] for i in range(m+1)]
    pppp = int(input())
    for i in range(pppp):
        a,b = map(int,stdin.readline().split())
        qw[b][a][1] = 1


    for i in range(m+1):
        for u in range(n+1):
            if u == i == 0:
                continue
            q = []
            if qw[i][u][1] == 1:


                q.append(qw[i-1][u-1][0]+(100**2+100**2)**0.5)
            if i != 0:


                q.append(qw[i-1][u][0]+100)
            if u != 0:


                q.append(qw[i][u-1][0]+100)
            qw[i][u][0] = min(q)
    print(int(round(min(q),0)))

kg()