# include <bits/stdc++.h>
# define rep (i, l, r) for (int (i) = (int)(l); (i) <= (int)(r); ++ (i))
# define ll long long

using namespace std;

int n, h = 0, len;
string s;

int main () {

	cin >> n >> s;
	len = s.size () - 1;
	rep (i, 0, len) {
		if (S[i] == 'I') h ++;
		else h --;
	}
	cout << h;

	return 0;

}

ABC052B，CE了。。。但是不知道怎么改，不太清楚宏定义。。。