Sudoku Subrectangles

#include<bits/stdc++.h>
#define inf 1e18
#define ll long long 
#define ull unsigned long long 
#define int long long
#define PI acos(-1.0)
#define PII pair<int,int>
using namespace std;
const int N =2e3+7 , M = 1e6+7;
const int mod = 1e9+7;
char s[N][N];
int r[N][N],d[N][N],re[N],n,m;
void solve(){
    scanf("%lld%lld",&n,&m);
    for(int i=1;i<=n;++i){
        scanf("%s",s[i]+1);
    }/*
    for(int i=1;i<=n;++i){
        printf("%s\n",s[i]+1);
    }*/
    map<char,int>las;
    for(int i=1;i<=n;++i){
        las.clear();
        for(int j=m;j>=1;--j){
            if(j==m){
                r[i][j]=m;
            }
            else{
                if(las[s[i][j]])r[i][j]=min(r[i][j+1],las[s[i][j]]-1);
                else            r[i][j]=r[i][j+1];
            }
            las[s[i][j]]=j;
        }
    }
    for(int j=1;j<=m;++j){
        las.clear();
        for(int i=n;i>=1;--i){
            if(i==n){
                d[i][j]=n;
            }
            else{
                if(las[s[i][j]])d[i][j]=min(d[i+1][j],las[s[i][j]]-1);
                else            d[i][j]=d[i+1][j];
            }
            las[s[i][j]]=i;
        }
    }
    int ans=0;
    for(int i=1;i<=n;++i){
        for(int j=1;j<=m;++j){
            int minn = d[i][j];
            for(int k = j; k<= r[i][j];++k){
                minn=min(minn,d[i][k]);
                re[k] = minn;
            }
            minn = r[i][j];
            for(int k = i;k <= d[i][j];++k){
                minn = min(minn, r[k][j]);
                while(re[minn] +1 <= k) --minn;
                ans += minn-j+1;
            }
        }
    }
    printf("%lld\n",ans);
}
signed main(){
    int t=1;
//    scanf("%lld",&t);
    while(t--){
        solve();
    }
    return 0;
}