import sys input = sys.stdin.readline def f(a, b, n): x = abs(a - b) return min(x, n - x) ans = 0 n, m = map(int, input().split()) l = [1]+[*map(int, input().split())] for i in range(1, m+1): ans += f(l[i - 1], l[i], n) print(ans)