If you are new to learning programming and do not know where to start, please try Problem A “Welcome to AtCoder” from practice contest. There you can find a sample code for each language.
Also, if you are not familiar with problems in programming contests, we recommend you to try some problems in “AtCoder Beginners Selection” (https://atcoder.jp/contests/abs).

This problem can be divided into the following two parts:

1. Count the number of occurrences of @ in \(S\). (Let \(A\) be this count.)
2. Evaluate \(N-S+D\) and print it.

Step 1 can be achieved with a loop structure (like a for statement) and a conditional branch (like an if statement); step 2 can be done with simple arithmetic operations.

For more details on implementation, please refer to the sample code below (C++ and Python).

Sample code (C++) :

#include <bits/stdc++.h>

using namespace std;

int main() {
    int n, d;
    string s;
    cin >> n >> d >> s;
    int a = 0;
    for (char c: s) {
        if (c == '@') ++a;
    }
    cout << n - a + d << endl;
}

Sample code (Python) :

n, d = map(int, input().split())
s = input()
a = 0
for c in s:
    if c == '@':
        a += 1
print(n - a + d)