n = int(input())
a = list(map(int,input().split(' ')))
ans = 0
for i in range(n):
    if i ==0 and a[0]==1:
        a[0],a[1] = a[1],a[0]
        ans += 1
        continue
    if i<n-1 and i+1 == a[i] and i+2 ==a[i+1]:
        a[i],a[i+1]  = a[i+1],a[i]
    if i+1 ==a[i] and i+1 != a[i-1]:
        a[i-1],a[i] = a[i],a[i-1]
        ans += 1
print(ans)