from sys import stdin
input = stdin.readline

def Hungarian(A):
    N = len(A)
    M = len(A[0])

    U = [0]*N
    V = [0]*M
    P = [0]*M
    way = [0]*M

    for i in range(1, N):
        P[0] = i
        minV = [INF]*M
        used = [False]*M
        j0 = 0
        while P[j0] != 0:
            i0 = P[j0]
            j1 = 0
            used[j0] = True
            delta = INF
            for j in range(1, M):
                if used[j]: continue
                curr = A[i0][j]-U[i0]-V[j]
                if curr < minV[j]:
                    minV[j] = curr
                    way[j] = j0
                if minV[j] < delta:
                    delta = minV[j]
                    j1 = j
            for j in range(M):
                if used[j]:
                    U[P[j]] += delta
                    V[j] -= delta
                else:
                    minV[j] -= delta
            j0 = j1
        while True:
            P[j0] = P[way[j0]]
            j0 = way[j0]
            if j0 == 0: break

    return -V[0]

INF = 1<<60

H, W = map(int, input().split())
XY = [list(map(int, input().split())) for _ in range(W)]

A = [[0]*(W+1) for _ in range(W+1)]
for i in range(W):
    for j, (h, w) in enumerate(XY):
        h, w = h-1, w-1
        cost = h
        if i < w-h:
            cost += (w-h)-i
        if w < i:
            cost += i-w
        A[i+1][j+1] = cost

print(Hungarian(A))