IOI '06 P1 - Deciphering the Mayan Writing

#include <iostream>
#include <string>

using namespace std;

inline int getID(char c) {
    return ('a' <= c && c <= 'z') ? c-'a' : c-'A'+26;
}

int main() {
    ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    int m, n; cin >> m >> n;
    string t, s; cin >> t >> s;
    int targetF[52]{}, f[52]{};
    for (char c : t) ++targetF[getID(c)];
    for (int i = 0; i < m-1; ++i) ++f[getID(s[i])];
    int differences = 0;
    for (int i = 0; i < 52; ++i) differences += targetF[i] != f[i];

    int total = 0;
    for (int i = m-1; i < n; ++i) {
        // add
        int c = getID(s[i]);
        ++f[c];
        if (f[c] == targetF[c]) --differences;
        else if (f[c] == targetF[c]+1) ++differences;

        // check
        if (!differences) ++total;

        // remove
        c = getID(s[i-m+1]);
        --f[c];
        if (f[c] == targetF[c]) --differences;
        else if (f[c] == targetF[c]-1) ++differences;
    }
    cout << total << endl;
    return 0;
}