#!/usr/bin/env python3
import sys


def solve(N: int, p: "List[int]"):

    ans = 0
    
    for i in range(N-1):
        if(i+1 == p[i]):
            p[i],p[i+1] = p[i+1],p[i]
            ans += 1

    # if(N == p[N-1]):
    #     ans += 1

    print(ans)

    # for i in range(N-1):
        
    #     if(i+1 == p[i]):
    #         p[i],p[i+1] = p[i+1],p[i]
    #         ans += 1

    # for i in range(N-1,-1,-1):
    #     if(i+1 == p[i]):
    #         p[i],p[i-1] = p[i-1],p[i]
    #         ans += 1    

    # print(ans)

    return


# Generated by 1.1.5 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    p = [int(next(tokens)) for _ in range(N)]  # type: "List[int]"
    solve(N, p)

if __name__ == '__main__':
    main()
