import std.algorithm, std.container, std.conv, std.math, std.range, std.typecons, std.stdio, std.string;

void readV(T...)(ref T t){auto r=readln.splitter;foreach(ref v;t){v=r.front.to!(typeof(v));r.popFront;}}
void readA(T)(size_t n,ref T[]t){t=new T[](n);auto r=readln.splitter;foreach(ref v;t){v=r.front.to!T;r.popFront;}}

void main()
{
  int n; readV(n);
  int[] p; readA(n, p);

  auto r = 0;
  foreach (i; 0..n-1)
    if (p[i] == i+1) {
      ++r;
      swap(p[i], p[i+1]);
    }

  if (p[n-1] == n)
    ++r;

  writeln(r);
}
