D.cpp

#include <bits/stdc++.h>

#define FOR(i,a,b) for(int i=(a),_b=(b); i<=_b; i++)
#define FORD(i,a,b) for(int i=(a),_b=(b); i>=_b; i--)
#define REP(i,a) for(int i=0,_a=(a); i<_a; i++)
#define EACH(it,a) for(__typeof(a.begin()) it = a.begin(); it != a.end(); ++it)

#define DEBUG(x) { cout << #x << " = "; cout << (x) << endl; }
#define PR(a,n) { cout << #a << " = "; FOR(_,1,n) cout << a[_] << ' '; cout << endl; }
#define PR0(a,n) { cout << #a << " = "; REP(_,n) cout << a[_] << ' '; cout << endl; }

#define sqr(x) ((x) * (x))
#define ll long long
#define SZ(X) ((int) ((X).size()))
using namespace std;

const int MN = 111;

char a[MN][MN];
int col[MN], m, n;

int main() {
    ios :: sync_with_stdio(false);
    while (cin >> m >> n) {
        FOR(i,1,m) FOR(j,1,n) cin >> a[i][j];

        FOR(i,1,n) col[i] = 0;

        int res = 0;
        FORD(i,m,1) FORD(j,n,1) {
            int cur = col[j];
            int need = (a[i][j] == 'B') ? -1 : 1;

            if (cur != need) {
                int add = need - cur;
                FOR(x,1,j) col[x] += add;

                ++res;
            }
        }
        cout << res << endl;
    }
}