n = int(input())
s = list(map(int, input()))
a = [i % 2 for i in range(n)]
b = [(i % 2) ^ 1 for i in range(n)]
ans1 = 0
ans2 = 0
for i in range(n):
    if s[i] == 0 and a[i] == 1: ans1 += 1
    if s[i] == 0 and b[i] == 1: ans2 += 1
print(min(ans1, ans2))