Adunare cifre matrice

#include <iostream.h>
int main()
{
	int m,n,a[20][30],sum,j,i;
	cout<<"m="; cin>>m;
	cout<<"n="; cin>>n;
	for(i=1;i<=m;i++)
		for(j=1;j<=n;j++){
			cout<<"a["<<i<<"]["<<j<<"]=";
			cin>>a[i][j];
		}
	cout<<"Matricea este \n";
	for(i=1;i<=m;i++){
		for(j=1;j<=n;j++)
			cout<<a[i][j]<<" ";
			cout<<endl;
	}
 sum=0;
	for(i=1;i<=m;i++){
		for(j=1;j<=n;j++)
		if(a[i][j]>=0)
				sum=sum+a[i][j];
	}
	cout<<sum;
}