#include<stdio.h>
#include<math.h>
#include<string.h>
#include<iostream>
#include<algorithm>
#include<queue>
#include<stack>
#include<set>
#include<map>
#include<vector>
#include<deque>
#include<string>
#define dint __int128
using namespace std;
#define kk putchar(32)
#define gc getchar()
#define fup(i,a,b) for(int i=a,i##e=b;i<=i##e;++i)
#define fdn(i,a,b) for(int i=a,i##e=b;i>=i##e;--i)
#define mem(a,b) memset(a,b,sizeof(a))
#define hh putchar(10)
#define ll long long
inline ll rrr(){ll x,f=1;char a;while((a=gc)<48||a>57)if(a==45)f=-1;
for(x=a^48;(a=gc)>47&&a<58;x=(x<<3)+(x<<1)+(a^48));return f*x;}
void Main();int main(){Main();return 0;}

void Main()
{
    int n;cin>>n;
    fup(i,1,n)a[i]=rrr();
    int ans=0;
    fup(i,1,n){
        if(a[i]==i)++ans,++i;
    }
    cout<<ans;
}


