import std.stdio, std.array, std.string, std.conv, std.algorithm;
import std.typecons, std.range, std.random, std.math, std.container;
import std.numeric, std.bigint, core.bitop;


void main() {
    auto N = readln.chomp.to!int;
    auto A = readln.split.map!(to!int).array;

    int ans = 0;
    for (int i = 0; i < N; i++) {
        if (A[i] == i + 1) {
            ans += 1;
            i += 1;
        }
    }

    ans.writeln;
}
