#include<iostream>
#include<string>
using namespace std;

int main(){

	int cnt, a, b, xl, xr, keta;
	string str, l1, l2, r;
  bool flg_zero;
  
  while(cin >> str){
  	cnt = 0;
  	l1 = "";
  	l2 = "";
  	r = "";
  	a = 0;
  	b = 0;
  	xl = 0;
  	xr = 0;
  	for(int i=0; i<str.length(); i++){
  		if(str[i] != '+' && str[i] != '='){
  			switch(cnt){
  				case 0: l1 += str[i]; break;
  				case 1: l2 += str[i]; break;
  				case 2: r += str[i]; break;
  			}
  		}
  		else cnt++;
  	}
  	
  	flg_zero = false;
  	if(l1.length() != 1 && l1[0] == 'X') flg_zero = true;
  	if(l2.length() != 1 && l2[0] == 'X') flg_zero = true;
  	if(r.length() != 1 && r[0] == 'X') flg_zero = true;
  	
  	keta = 1;
  	for(int i=l1.length() - 1; i>=0; i--){
  		if(l1[i] != 'X'){
  			a += (l1[i] - '0') * keta;
  		}
  		else xl += keta;
  		keta *= 10;
  	}

  	keta = 1;
  	for(int i=l2.length() - 1; i>=0; i--){
  		if(l2[i] != 'X'){
  			a += (l2[i] - '0') * keta;
  		}
  		else xl += keta;
  		keta *= 10;
  	}

  	keta = 1;
  	for(int i=r.length() - 1; i>=0; i--){
  		if(r[i] != 'X'){
  			b += (r[i] - '0') * keta;
  		}
  		else xr += keta;
  		keta *= 10;
  	}

  	int i;
  	if(flg_zero != true) i = 0;
  	else i = 1;
  	for(; i<10; i++){
  		if(a + xl * i == b + xr * i){
  			cout << i << endl;
  			break;
  		}
  		if(i == 9){
  			cout << "NA" << endl;
  		}
  	}
  	
  }

  return 0;

}