from collections import deque

INF = 10 ** 18

N, M = map(int, input().split())

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

dp0 = [[(INF, 0)] * N for _ in range(N)]
dp1 = [[(INF, 0)] * N for _ in range(N)]
dp0[0][0] = (0, 0)
q = deque([(0, 0, 0)])
while q:
    h, w, t = q.popleft()
    if t == 0:
        if h != 0:
            if dp0[h - 1][w] > dp0[h][w] + 1 + c[h - 1][w]:
                dp0[h - 1][w] = dp0[h][w] + 1 + c[h - 1][w]
                q.append((h - 1, w, 0))
            if dp1[h - 1][w] > dp0[h][w] + 1:
                dp0[h - 1][w] = dp0[h][w] + 1
                q.append((h - 1, w, 1))
        if h != N - 1:
            if dp0[h + 1][w] > dp0[h][w] + 1 + c[h + 1][w]:
                dp0[h + 1][w] = dp0[h][w] + 1 + c[h + 1][w]
                q.append((h + 1, w, 0))
            if dp1[h + 1][w] > dp0[h][w] + 1:
                dp0[h + 1][w] = dp0[h][w] + 1
                q.append((h + 1, w, 1))
        if w != 0:
            if dp0[h][w - 1] > dp0[h][w] + 1 + c[h][w - 1]:
                dp0[h][w - 1] = dp0[h][w] + 1 + c[h][w - 1]
                q.append((h, w - 1, 0))
            if dp1[h][w - 1] > dp0[h][w] + 1:
                dp0[h][w - 1] = dp0[h][w] + 1
                q.append((h, w - 1, 1))
        if w != N - 1:
            if dp0[h][w + 1] > dp0[h][w] + 1 + c[h][w + 1]:
                dp0[h][w + 1] = dp0[h][w] + 1 + c[h][w + 1]
                q.append((h, w + 1, 0))
            if dp1[h][w + 1] > dp0[h][w] + 1:
                dp0[h][w + 1] = dp0[h][w] + 1
                q.append((h, w + 1, 1))
    elif t == 1:
        if h != 0:
            if dp1[h - 1][w] > dp1[h][w] + 1 + c[h - 1][w]:
                dp1[h - 1][w] = dp1[h][w] + 1 + c[h - 1][w]
                q.append((h - 1, w, 0))
        if h != N - 1:
            if dp1[h + 1][w] > dp1[h][w] + 1 + c[h + 1][w]:
                dp1[h + 1][w] = dp1[h][w] + 1 + c[h + 1][w]
                q.append((h + 1, w, 0))
        if w != 0:
            if dp1[h][w - 1] > dp1[h][w] + 1 + c[h][w - 1]:
                dp1[h][w - 1] = dp1[h][w] + 1 + c[h][w - 1]
                q.append((h, w - 1, 0))
        if w != N - 1:
            if dp1[h][w + 1] > dp1[h][w] + 1 + c[h][w + 1]:
                dp1[h][w + 1] = dp1[h][w] + 1 + c[h][w + 1]
                q.append((h, w + 1, 0))
print(min(dp0[N - 1][N - 1], dp1[N - 1][N - 1]))