def main():
    import sys
    from heapq import heappush,heappop
    input = sys.stdin.readline

    N, M = map(int, input().split())
    cost = [[0 for _ in range(N)] for _ in range(N)]
    for i in range(M):
        h, w, c = map(int, input().split())
        h-=1;w-=1
        cost[h][w] = c

    dis1 = [[float('inf') for _ in range(N)] for _ in range(N)]
    dis2 = [[float('inf') for _ in range(N)] for _ in range(N)]
    dis1[0][0] = 0
    dis2[0][0] = 0
    p = [(0,0,0,0)]
    xx = [0,1,0,-1]
    yy = [1,0,-1,0]
    while p:
        c, x, y, f = heappop(p)
        if f==0 and dis1[x][y]<c:
            continue
        if f==1 and dis2[x][y]<c:
            continue
        for px, py in zip(xx, yy):
            cx = x+px
            cy = y+py
            if 0<=cx<N and 0<=cy<N:
                cost1 = c+cost[cx][cy]+1
                cost2 = c+1
                if f:
                    if dis2[cx][cy]>cost1:
                        dis2[cx][cy] = cost1
                        heappush(p, (cost1, cx, cy, f))
                else:
                    if dis1[cx][cy]>cost1:
                        dis1[cx][cy] = cost1
                        heappush(p, (cost1, cx, cy, f))
                    if dis2[cx][cy]>cost2:
                        dis2[cx][cy] = cost2
                        heappush(p, (cost2, cx, cy, f+1))

    print(min(dis1[-1][-1],dis2[-1][-1]))
if __name__=='__main__':
    main()